diff --git a/docs-developer/CHANGELOG-formats.md b/docs-developer/CHANGELOG-formats.md
index e07d501aa6..b59d5fe0e3 100644
--- a/docs-developer/CHANGELOG-formats.md
+++ b/docs-developer/CHANGELOG-formats.md
@@ -6,6 +6,10 @@ Note that this is not an exhaustive list. Processed profile format upgraders can
 
 ## Processed profile format
 
+### Version 65
+
+The stack table's `frame` column (stored at `profile.shared.stackTable.frame`) can now optionally be stored as an `Int32Array`, for profiles loaded from [JsonSlabs](https://github.com/mstange/json-slabs/) files (.jslb, .jslb.gz). Regular JS / JSON arrays are still accepted.
+
 ### Version 64
 
 A new `SourceLocationTable` has been added to `profile.shared.sourceLocationTable`. It holds the original (pre-compilation) source positions produced by source map symbolication, paired with the generated `line`/`column` already on `FrameTable`.
diff --git a/src/app-logic/constants.ts b/src/app-logic/constants.ts
index 6cba739b59..8d39768163 100644
--- a/src/app-logic/constants.ts
+++ b/src/app-logic/constants.ts
@@ -12,7 +12,7 @@ export const GECKO_PROFILE_VERSION = 34;
 // The current version of the "processed" profile format.
 // Please don't forget to update the processed profile format changelog in
 // `docs-developer/CHANGELOG-formats.md`.
-export const PROCESSED_PROFILE_VERSION = 64;
+export const PROCESSED_PROFILE_VERSION = 65;
 
 // The following are the margin sizes for the left and right of the timeline. Independent
 // components need to share these values.
diff --git a/src/profile-logic/data-structures.ts b/src/profile-logic/data-structures.ts
index 18f8ab12fc..0599e2153e 100644
--- a/src/profile-logic/data-structures.ts
+++ b/src/profile-logic/data-structures.ts
@@ -27,6 +27,8 @@ import type {
   CallNodeTable,
   SourceTable,
   SourceLocationTable,
+  IndexIntoFrameTable,
+  IndexIntoStackTable,
 } from 'firefox-profiler/types';
 
 /**
@@ -47,7 +49,13 @@ export function getEmptySamplesTable(): RawSamplesTable {
   };
 }
 
-export function getEmptyRawStackTable(): RawStackTable {
+export type RawStackTableBuilder = {
+  frame: IndexIntoFrameTable[];
+  prefix: Array<IndexIntoStackTable | null>;
+  length: number;
+};
+
+export function getRawStackTableBuilder(): RawStackTableBuilder {
   return {
     // Important!
     // If modifying this structure, please update all callers of this function to ensure
@@ -59,6 +67,27 @@ export function getEmptyRawStackTable(): RawStackTable {
   };
 }
 
+export function getRawStackTableBuilderWithExistingContents(
+  existing: RawStackTable
+): RawStackTableBuilder {
+  return {
+    frame: [...existing.frame],
+    prefix: [...existing.prefix],
+    length: existing.length,
+  };
+}
+
+export function finishRawStackTableBuilder(
+  builder: RawStackTableBuilder
+): RawStackTable {
+  const { frame, prefix, length } = builder;
+  return {
+    frame: new Int32Array(frame),
+    prefix,
+    length,
+  };
+}
+
 /**
  * Returns an empty samples table with eventDelay field instead of responsiveness.
  * eventDelay is a new field and it replaced responsiveness. We should still
@@ -393,7 +422,7 @@ export function getEmptyThread(overrides?: Partial<RawThread>): RawThread {
 
 export function getEmptySharedData(): RawProfileSharedData {
   return {
-    stackTable: getEmptyRawStackTable(),
+    stackTable: finishRawStackTableBuilder(getRawStackTableBuilder()),
     frameTable: getEmptyFrameTable(),
     funcTable: getEmptyFuncTable(),
     resourceTable: getEmptyResourceTable(),
diff --git a/src/profile-logic/global-data-collector.ts b/src/profile-logic/global-data-collector.ts
index 78b5b38543..c52e170293 100644
--- a/src/profile-logic/global-data-collector.ts
+++ b/src/profile-logic/global-data-collector.ts
@@ -4,13 +4,14 @@
 
 import { StringTable } from '../utils/string-table';
 import {
+  finishRawStackTableBuilder,
   getEmptyFrameTable,
   getEmptyFuncTable,
   getEmptyNativeSymbolTable,
-  getEmptyRawStackTable,
   getEmptyResourceTable,
   getEmptySourceTable,
   getEmptySourceLocationTable,
+  getRawStackTableBuilder,
 } from './data-structures';
 
 import type {
@@ -22,7 +23,6 @@ import type {
   RawProfileSharedData,
   SourceTable,
   FrameTable,
-  RawStackTable,
   FuncTable,
   ResourceTable,
   NativeSymbolTable,
@@ -34,6 +34,7 @@ import type {
   Bytes,
 } from 'firefox-profiler/types';
 import { ResourceType } from 'firefox-profiler/types';
+import type { RawStackTableBuilder } from './data-structures';
 
 /**
  * GlobalDataCollector collects data which is global in the processed profile
@@ -50,7 +51,7 @@ export class GlobalDataCollector {
   _stringTable: StringTable = StringTable.withBackingArray(this._stringArray);
   _sources: SourceTable = getEmptySourceTable();
   _frameTable: FrameTable = getEmptyFrameTable();
-  _stackTable: RawStackTable = getEmptyRawStackTable();
+  _stackTableBuilder: RawStackTableBuilder = getRawStackTableBuilder();
   _funcTable: FuncTable = getEmptyFuncTable();
   _resourceTable: ResourceTable = getEmptyResourceTable();
   _nativeSymbols: NativeSymbolTable = getEmptyNativeSymbolTable();
@@ -302,15 +303,15 @@ export class GlobalDataCollector {
     return this._frameTable;
   }
 
-  getStackTable(): RawStackTable {
-    return this._stackTable;
+  getStackTableBuilder(): RawStackTableBuilder {
+    return this._stackTableBuilder;
   }
 
   // Package up all de-duplicated global tables so that they can be embedded in
   // the profile.
   finish(): { libs: Lib[]; shared: RawProfileSharedData } {
     const shared: RawProfileSharedData = {
-      stackTable: this._stackTable,
+      stackTable: finishRawStackTableBuilder(this._stackTableBuilder),
       frameTable: this._frameTable,
       funcTable: this._funcTable,
       resourceTable: this._resourceTable,
diff --git a/src/profile-logic/import/chrome.ts b/src/profile-logic/import/chrome.ts
index 96add0a137..be4253f0ce 100644
--- a/src/profile-logic/import/chrome.ts
+++ b/src/profile-logic/import/chrome.ts
@@ -501,7 +501,7 @@ async function processTracingEvents(
   const stringTable = globalDataCollector.getStringTable();
 
   const frameTable = globalDataCollector.getFrameTable();
-  const stackTable = globalDataCollector.getStackTable();
+  const stackTable = globalDataCollector.getStackTableBuilder();
 
   let profileEvents: (ProfileEvent | CpuProfileEvent)[] = (eventsByName.get(
     'Profile'
diff --git a/src/profile-logic/import/dhat.ts b/src/profile-logic/import/dhat.ts
index dc56c06f17..6f883c2e29 100644
--- a/src/profile-logic/import/dhat.ts
+++ b/src/profile-logic/import/dhat.ts
@@ -183,7 +183,7 @@ export function attemptToConvertDhat(json: unknown): Profile | null {
   profile.meta.product = dhat.cmd + ' (dhat)';
   profile.meta.importedFrom = `dhat`;
   const globalDataCollector = new GlobalDataCollector();
-  const stackTable = globalDataCollector.getStackTable();
+  const stackTable = globalDataCollector.getStackTableBuilder();
   const frameTable = globalDataCollector.getFrameTable();
   const stringTable = globalDataCollector.getStringTable();
 
diff --git a/src/profile-logic/import/flame-graph.ts b/src/profile-logic/import/flame-graph.ts
index 148a777bd2..b468d4d008 100644
--- a/src/profile-logic/import/flame-graph.ts
+++ b/src/profile-logic/import/flame-graph.ts
@@ -60,7 +60,7 @@ export function convertFlameGraphProfile(profileText: string): Profile {
   });
 
   const frameTable = globalDataCollector.getFrameTable();
-  const stackTable = globalDataCollector.getStackTable();
+  const stackTable = globalDataCollector.getStackTableBuilder();
   const { samples } = thread;
 
   // Maps to deduplicate stacks, frames, and functions.
diff --git a/src/profile-logic/import/simpleperf.ts b/src/profile-logic/import/simpleperf.ts
index 89b632cd85..83c38d8929 100644
--- a/src/profile-logic/import/simpleperf.ts
+++ b/src/profile-logic/import/simpleperf.ts
@@ -24,7 +24,9 @@ import {
   getEmptyFuncTable,
   getEmptyResourceTable,
   getEmptyFrameTable,
-  getEmptyRawStackTable,
+  getRawStackTableBuilder,
+  finishRawStackTableBuilder,
+  type RawStackTableBuilder,
   getEmptySamplesTable,
   getEmptyRawMarkerTable,
   getEmptyNativeSymbolTable,
@@ -189,7 +191,7 @@ class FirefoxFrameTable {
 class FirefoxSampleTable {
   strings: StringTable;
 
-  stackTable: RawStackTable = getEmptyRawStackTable();
+  stackTable: RawStackTableBuilder = getRawStackTableBuilder();
   stackMap: Map<string, IndexIntoStackTable> = new Map();
 
   constructor(strings: StringTable) {
@@ -197,7 +199,7 @@ class FirefoxSampleTable {
   }
 
   toJson(): RawStackTable {
-    return this.stackTable;
+    return finishRawStackTableBuilder(this.stackTable);
   }
 
   findOrAddStack(
diff --git a/src/profile-logic/js-tracer.ts b/src/profile-logic/js-tracer.ts
index 8993ddfbca..e4664aee95 100644
--- a/src/profile-logic/js-tracer.ts
+++ b/src/profile-logic/js-tracer.ts
@@ -4,6 +4,8 @@
 import {
   getEmptySamplesTableWithEventDelay,
   getEmptyRawMarkerTable,
+  finishRawStackTableBuilder,
+  getRawStackTableBuilderWithExistingContents,
 } from './data-structures';
 import { StringTable } from '../utils/string-table';
 import { ensureExists } from '../utils/types';
@@ -512,7 +514,10 @@ export function convertJsTracerToThreadWithoutSamples(
     samples,
   };
 
-  const { funcTable, frameTable, stackTable } = shared;
+  const { funcTable, frameTable } = shared;
+  const stackTable = getRawStackTableBuilderWithExistingContents(
+    shared.stackTable
+  );
 
   // Keep a stack of js tracer events, and end timings, that will be used to find
   // the stack prefixes. Once a JS tracer event starts past another event end, the
@@ -620,6 +625,9 @@ export function convertJsTracerToThreadWithoutSamples(
     unmatchedEventEnds[unmatchedIndex] = end;
   }
 
+  // Write the augmented stackTable back to the shared data.
+  shared.stackTable = finishRawStackTableBuilder(stackTable);
+
   return { thread, stackMap };
 }
 
diff --git a/src/profile-logic/merge-compare.ts b/src/profile-logic/merge-compare.ts
index 5eeecb1705..6e9e424b1b 100644
--- a/src/profile-logic/merge-compare.ts
+++ b/src/profile-logic/merge-compare.ts
@@ -13,7 +13,8 @@ import {
   getEmptyNativeSymbolTable,
   getEmptyFrameTable,
   getEmptyFuncTable,
-  getEmptyRawStackTable,
+  getRawStackTableBuilder,
+  finishRawStackTableBuilder,
   getEmptyRawMarkerTable,
   getEmptySamplesTableWithEventDelay,
   shallowCloneRawMarkerTable,
@@ -1117,7 +1118,7 @@ function mergeStackTables(
   translationMapsForFrames: TranslationMapForFrames[]
 ): { stackTable: RawStackTable; translationMaps: TranslationMapForStacks[] } {
   const translationMaps: TranslationMapForStacks[] = [];
-  const newStackTable = getEmptyRawStackTable();
+  const newStackTable = getRawStackTableBuilder();
 
   profiles.forEach((profile, profileIndex) => {
     const { stackTable } = profile.shared;
@@ -1143,7 +1144,10 @@ function mergeStackTables(
     translationMaps.push(oldStackToNewStackPlusOne);
   });
 
-  return { stackTable: newStackTable, translationMaps };
+  return {
+    stackTable: finishRawStackTableBuilder(newStackTable),
+    translationMaps,
+  };
 }
 
 /**
diff --git a/src/profile-logic/process-profile.ts b/src/profile-logic/process-profile.ts
index 712dda0c52..d1f96c4c90 100644
--- a/src/profile-logic/process-profile.ts
+++ b/src/profile-logic/process-profile.ts
@@ -16,6 +16,7 @@ import {
   getEmptyRawMarkerTable,
   getEmptyJsAllocationsTable,
   getEmptyUnbalancedNativeAllocationsTable,
+  type RawStackTableBuilder,
 } from './data-structures';
 import { immutableUpdate, ensureExists } from '../utils/types';
 import { verifyMagic, SIMPLEPERF as SIMPLEPERF_MAGIC } from '../utils/magic';
@@ -53,7 +54,6 @@ import type {
   FrameTable,
   RawCounterSamplesTable,
   RawSamplesTable,
-  RawStackTable,
   RawMarkerTable,
   LibMapping,
   IndexIntoStackTable,
@@ -107,6 +107,7 @@ import type {
   CounterDisplayConfig,
 } from 'firefox-profiler/types';
 import { decompress, isGzip } from 'firefox-profiler/utils/gz';
+import { jsonEncodeObjectWithTypedArraysAsRegularArrays } from 'firefox-profiler/utils/json-with-typed-arrays';
 
 type RegExpResult = null | string[];
 /**
@@ -548,7 +549,7 @@ function _processFrameTable(
  */
 function _processStackTable(
   geckoStackTable: GeckoStackStruct,
-  sharedStackTable: RawStackTable,
+  sharedStackTable: RawStackTableBuilder,
   frameIndexOffset: IndexIntoFrameTable
 ): IndexIntoStackTable {
   const stackIndexOffset = sharedStackTable.length;
@@ -1336,7 +1337,7 @@ function _processThread(
   );
   const stackIndexOffset = _processStackTable(
     geckoStackTable,
-    globalDataCollector.getStackTable(),
+    globalDataCollector.getStackTableBuilder(),
     frameIndexOffset
   );
 
@@ -2056,7 +2057,7 @@ export function processGeckoProfile(geckoProfile: GeckoProfile): Profile {
  * Take a processed profile and convert it to a string.
  */
 export function serializeProfileToJsonString(profile: Profile): string {
-  return JSON.stringify(profile);
+  return jsonEncodeObjectWithTypedArraysAsRegularArrays(profile);
 }
 
 /**
diff --git a/src/profile-logic/processed-profile-versioning.ts b/src/profile-logic/processed-profile-versioning.ts
index bcf3931179..16ab69e409 100644
--- a/src/profile-logic/processed-profile-versioning.ts
+++ b/src/profile-logic/processed-profile-versioning.ts
@@ -3255,6 +3255,11 @@ const _upgraders: {
     };
     sources.content = new Array(sources.length).fill(null);
   },
+  [65]: (_profile: any) => {
+    // The type of `profile.shared.stackTable.frame` was changed from
+    // `IndexIntoFrameTable[]` to `IndexIntoFrameTable[] | Int32Array<ArrayBuffer>`.
+    // All valid v64 profiles are valid v65 profiles, so no upgrader is needed.
+  },
   // If you add a new upgrader here, please document the change in
   // `docs-developer/CHANGELOG-formats.md`.
 };
diff --git a/src/profile-logic/profile-compacting.ts b/src/profile-logic/profile-compacting.ts
index 492f914716..e7300a3551 100644
--- a/src/profile-logic/profile-compacting.ts
+++ b/src/profile-logic/profile-compacting.ts
@@ -48,15 +48,22 @@ type ColumnDescription<TCol> = null extends (
       | { type: 'INDEX_REF_OR_NULL'; referencedTable: TableCompactionState }
       | { type: 'SELF_INDEX_REF_OR_NULL' }
       | { type: 'NO_REF' }
-  :
-      | { type: 'INDEX_REF'; referencedTable: TableCompactionState }
-      | { type: 'INDEX_REF_OR_NEG_ONE'; referencedTable: TableCompactionState }
-      | { type: 'NO_REF' };
+  : Int32Array<ArrayBuffer> extends TCol
+    ?
+        | { type: 'INDEX_REF_INT32'; referencedTable: TableCompactionState }
+        | { type: 'NO_REF' }
+    :
+        | { type: 'INDEX_REF'; referencedTable: TableCompactionState }
+        | {
+            type: 'INDEX_REF_OR_NEG_ONE';
+            referencedTable: TableCompactionState;
+          }
+        | { type: 'NO_REF' };
 
 type TableDescription<T> = {
-  [K in keyof T as T[K] extends Array<any> ? K : never]: ColumnDescription<
-    T[K]
-  >;
+  [K in keyof T as T[K] extends Array<any> | Int32Array<ArrayBuffer>
+    ? K
+    : never]: ColumnDescription<T[K]>;
 };
 
 const ColDesc = {
@@ -64,6 +71,10 @@ const ColDesc = {
     type: 'INDEX_REF' as const,
     referencedTable,
   }),
+  indexRefInt32: (referencedTable: TableCompactionState) => ({
+    type: 'INDEX_REF_INT32' as const,
+    referencedTable,
+  }),
   indexRefOrNull: (referencedTable: TableCompactionState) => ({
     type: 'INDEX_REF_OR_NULL' as const,
     referencedTable,
@@ -145,7 +156,7 @@ export function computeCompactedProfile(
   };
 
   const stackTableDesc: TableDescription<RawStackTable> = {
-    frame: ColDesc.indexRef(tcs.frameTable),
+    frame: ColDesc.indexRefInt32(tcs.frameTable),
     prefix: ColDesc.selfIndexRefOrNull(),
   };
   const frameTableDesc: TableDescription<FrameTable> = {
@@ -326,6 +337,7 @@ function _markTableAndComputeTranslation<T>(
     const col = (table as any)[key];
     switch (desc.type) {
       case 'INDEX_REF':
+      case 'INDEX_REF_INT32':
         markColumn(col, markBuffer, desc.referencedTable.markBuffer);
         break;
       case 'INDEX_REF_OR_NULL':
@@ -354,7 +366,13 @@ function _markTableAndComputeTranslation<T>(
   thisTableCompactionState.computeIndexTranslation();
 }
 
-function markColumn(col: Array<number>, shouldMark: BitSet, markBuf: BitSet) {
+function markColumn(
+  col: Array<number> | Int32Array<ArrayBuffer>,
+  shouldMark: BitSet,
+  markBuf: BitSet
+) {
+  // Polymorphic: indexing works the same on Int32Array as on number[], so the
+  // INDEX_REF and INDEX_REF_INT32 cases share this function.
   for (let i = 0; i < col.length; i++) {
     if (checkBit(shouldMark, i)) {
       const val = col[i];
@@ -499,6 +517,14 @@ function _compactTable<T extends { length: number }>(
           newLength
         );
         break;
+      case 'INDEX_REF_INT32':
+        result[key] = _compactColIndexInt32(
+          oldCol,
+          markBuffer,
+          desc.referencedTable.oldIndexToNewIndexPlusOne,
+          newLength
+        );
+        break;
       case 'INDEX_REF_OR_NULL':
         result[key] = _compactColIndexOrNull(
           oldCol,
@@ -564,6 +590,22 @@ function _compactColIndex(
   return newCol;
 }
 
+function _compactColIndexInt32(
+  oldCol: Int32Array<ArrayBuffer>,
+  markBuffer: BitSet,
+  oldIndexToNewIndexPlusOne: Int32Array,
+  newLength: number
+): Int32Array<ArrayBuffer> {
+  const newCol = new Int32Array(newLength);
+  let newIndex = 0;
+  for (let i = 0; i < oldCol.length; i++) {
+    if (checkBit(markBuffer, i)) {
+      newCol[newIndex++] = oldIndexToNewIndexPlusOne[oldCol[i]] - 1;
+    }
+  }
+  return newCol;
+}
+
 function _compactColIndexOrNull(
   oldCol: (number | null)[],
   markBuffer: BitSet,
diff --git a/src/profile-logic/profile-data.ts b/src/profile-logic/profile-data.ts
index 787fdc7fda..3b6e424499 100644
--- a/src/profile-logic/profile-data.ts
+++ b/src/profile-logic/profile-data.ts
@@ -6,7 +6,8 @@ import memoize from 'memoize-immutable';
 import MixedTupleMap from 'mixedtuplemap';
 import { oneLine } from 'common-tags';
 import {
-  getEmptyRawStackTable,
+  getRawStackTableBuilder,
+  finishRawStackTableBuilder,
   getEmptyCallNodeTable,
   shallowCloneFrameTable,
   shallowCloneFuncTable,
@@ -1688,7 +1689,7 @@ export function createStackTableBySkippingDiscarded(
   keepStack: BitSet
 ): StackTable {
   const newStackCount = newPrefixCol.length;
-  const newFrameCol = new Array<IndexIntoFrameTable>(newStackCount);
+  const newFrameCol = new Int32Array(newStackCount);
   const newCategoryCol = new Uint8Array(newStackCount);
   const newSubcategoryCol =
     stackTable.subcategory instanceof Uint16Array
@@ -4343,7 +4344,7 @@ export function nudgeReturnAddresses(profile: Profile): Profile {
   // Now the frame table contains adjusted / "nudged" addresses.
 
   // Make a new stack table which refers to the adjusted frames.
-  const newStackTable = getEmptyRawStackTable();
+  const newStackTable = getRawStackTableBuilder();
   const mapForSamplingSelfStacks = new Map<
     null | IndexIntoStackTable,
     null | IndexIntoStackTable
@@ -4385,7 +4386,7 @@ export function nudgeReturnAddresses(profile: Profile): Profile {
   const newShared: RawProfileSharedData = {
     ...profile.shared,
     frameTable: newFrameTable,
-    stackTable: newStackTable,
+    stackTable: finishRawStackTableBuilder(newStackTable),
   };
 
   const newThreads = updateRawThreadStacksSeparate(
@@ -4764,8 +4765,14 @@ export function computeStackTableFromRawStackTable(
     subcategoryColumn[stackIndex] = stackSubcategory;
   }
 
+  // The frame column is a typed array in the derived stack table.
+  const frame =
+    rawStackTable.frame instanceof Int32Array
+      ? rawStackTable.frame
+      : new Int32Array(rawStackTable.frame);
+
   return {
-    frame: rawStackTable.frame,
+    frame,
     category: categoryColumn,
     subcategory: subcategoryColumn,
     prefix: rawStackTable.prefix,
diff --git a/src/profile-logic/symbolication.ts b/src/profile-logic/symbolication.ts
index a6370bc6fa..dfbfd345c2 100644
--- a/src/profile-logic/symbolication.ts
+++ b/src/profile-logic/symbolication.ts
@@ -2,7 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 import {
-  getEmptyRawStackTable,
+  getRawStackTableBuilder,
+  finishRawStackTableBuilder,
   shallowCloneFuncTable,
   shallowCloneNativeSymbolTable,
   shallowCloneFrameTable,
@@ -420,7 +421,7 @@ function _computeStackTableWithAddedExpansionStacks(
   if (frameIndexToInlineExpansionFrames.size === 0) {
     return null;
   }
-  const newStackTable = getEmptyRawStackTable();
+  const newStackTable = getRawStackTableBuilder();
   const oldStackToNewStack = new Int32Array(stackTable.length);
   for (let stack = 0; stack < stackTable.length; stack++) {
     const oldFrame = stackTable.frame[stack];
@@ -452,7 +453,10 @@ function _computeStackTableWithAddedExpansionStacks(
     }
     oldStackToNewStack[stack] = prefix ?? -1;
   }
-  return { newStackTable, oldStackToNewStack };
+  return {
+    newStackTable: finishRawStackTableBuilder(newStackTable),
+    oldStackToNewStack,
+  };
 }
 
 /**
diff --git a/src/test/components/Root-history.test.tsx b/src/test/components/Root-history.test.tsx
index 13a130aa71..adacbd6510 100644
--- a/src/test/components/Root-history.test.tsx
+++ b/src/test/components/Root-history.test.tsx
@@ -13,6 +13,7 @@ import { autoMockCanvasContext } from '../fixtures/mocks/canvas-context';
 import { fireFullClick } from '../fixtures/utils';
 import { getProfileUrlForHash } from '../../utils/profile-fetch';
 import { blankStore } from '../fixtures/stores';
+import { serializeProfileToJsonString } from '../../profile-logic/process-profile';
 import { getProfileFromTextSamples } from '../fixtures/profiles/processed-profile';
 import {
   autoMockFullNavigation,
@@ -198,5 +199,5 @@ describe('Root with history', function () {
 function mockFetchProfileAtUrl(url: string, profile: Profile): void {
   window.fetchMock
     .catch(404) // catchall
-    .get(url, profile);
+    .get(url, serializeProfileToJsonString(profile));
 }
diff --git a/src/test/components/UrlManager.test.tsx b/src/test/components/UrlManager.test.tsx
index 86d9f4ef3d..e5811d062a 100644
--- a/src/test/components/UrlManager.test.tsx
+++ b/src/test/components/UrlManager.test.tsx
@@ -18,6 +18,7 @@ import { waitUntilState } from '../fixtures/utils';
 import { createGeckoProfile } from '../fixtures/profiles/gecko-profile';
 import { getProfileFromTextSamples } from '../fixtures/profiles/processed-profile';
 import { CURRENT_URL_VERSION } from '../../app-logic/url-handling';
+import { serializeProfileToJsonString } from '../../profile-logic/process-profile';
 import { autoMockFullNavigation } from '../fixtures/mocks/window-navigation';
 import { profilePublished } from 'firefox-profiler/actions/publish';
 import {
@@ -35,7 +36,7 @@ describe('UrlManager', function () {
   autoMockFullNavigation();
 
   function getSerializableProfile() {
-    return getProfileFromTextSamples('A').profile;
+    return serializeProfileToJsonString(getProfileFromTextSamples('A').profile);
   }
 
   function setup(urlPath?: string) {
diff --git a/src/test/fixtures/profiles/call-nodes.ts b/src/test/fixtures/profiles/call-nodes.ts
index 909b48e7e7..0ffed2dd52 100644
--- a/src/test/fixtures/profiles/call-nodes.ts
+++ b/src/test/fixtures/profiles/call-nodes.ts
@@ -6,7 +6,8 @@ import type { FuncTable, FrameTable, Profile } from 'firefox-profiler/types';
 import {
   getEmptyThread,
   getEmptyProfile,
-  getEmptyRawStackTable,
+  getRawStackTableBuilder,
+  finishRawStackTableBuilder,
 } from '../../../profile-logic/data-structures';
 import { StringTable } from '../../../utils/string-table';
 
@@ -87,7 +88,7 @@ export default function getProfile(): Profile {
     length: frameFuncs.length,
   };
 
-  const stackTable = getEmptyRawStackTable();
+  const stackTable = getRawStackTableBuilder();
 
   // Provide a utility function for readability.
   function addToStackTable(frame: any, prefix: any) {
@@ -119,7 +120,12 @@ export default function getProfile(): Profile {
     length: 2,
   };
 
-  profile.shared = { ...profile.shared, stackTable, funcTable, frameTable };
+  profile.shared = {
+    ...profile.shared,
+    stackTable: finishRawStackTableBuilder(stackTable),
+    funcTable,
+    frameTable,
+  };
 
   profile.threads.push(thread, { ...thread }, { ...thread });
 
diff --git a/src/test/fixtures/profiles/processed-profile.ts b/src/test/fixtures/profiles/processed-profile.ts
index 07ef1d7bab..708018511f 100644
--- a/src/test/fixtures/profiles/processed-profile.ts
+++ b/src/test/fixtures/profiles/processed-profile.ts
@@ -8,6 +8,8 @@ import {
   getEmptyJsAllocationsTable,
   getEmptyUnbalancedNativeAllocationsTable,
   getEmptyBalancedNativeAllocationsTable,
+  getRawStackTableBuilderWithExistingContents,
+  finishRawStackTableBuilder,
 } from '../../../profile-logic/data-structures';
 import { mergeProfilesForDiffing } from '../../../profile-logic/merge-compare';
 import { computeReferenceCPUDeltaPerMs } from '../../../profile-logic/cpu';
@@ -983,7 +985,7 @@ function _buildThreadFromTextOnlyStacks(
       }
 
       // Attempt to find a stack that satisfies the given frameIndex and prefix.
-      const stackTable = globalDataCollector.getStackTable();
+      const stackTable = globalDataCollector.getStackTableBuilder();
       let stackIndex;
       for (let i = 0; i < stackTable.length; i++) {
         if (
@@ -1994,8 +1996,10 @@ function getStackIndexForCallNodePath(
 /**
  * Use this function to add window id information to frames, using call node
  * paths to point to frames using stacks.
+ * This mutates `shared`.
  *
  * @param thread The thread to mutate.
+ * @param shared The shared profile data to mutate.
  * @param listOfOperations A list of pairs { innerWindowID, callNodes }
  *                         indicating which call nodes this innerWindowID will
  *                         be assigned to.
@@ -2037,6 +2041,9 @@ export function addInnerWindowIdToStacks(
       IndexIntoStackTable
     >();
 
+    const stackTableBuilder =
+      getRawStackTableBuilderWithExistingContents(stackTable);
+
     for (const callNode of callNodesToDupe) {
       const stackIndex = getStackIndexForCallNodePath(shared, callNode);
       const foundFrameIndex = stackTable.frame[stackIndex];
@@ -2062,14 +2069,16 @@ export function addInnerWindowIdToStacks(
       frameTable.innerWindowID.push(listOfOperations[1].innerWindowID);
 
       // Clone the stack
-      const newStackIndex = stackTable.length++;
-      stackTable.prefix.push(stackTable.prefix[stackIndex]);
+      const newStackIndex = stackTableBuilder.length++;
+      stackTableBuilder.prefix.push(stackTable.prefix[stackIndex]);
       // Using the cloned frame index.
-      stackTable.frame.push(newFrameIndex);
+      stackTableBuilder.frame.push(newFrameIndex);
 
       mapStackIndexToDupe.set(stackIndex, newStackIndex);
     }
 
+    shared.stackTable = finishRawStackTableBuilder(stackTableBuilder);
+
     const sampleTimes = ensureExists(samples.time);
     for (let sampleIndex = samples.length; sampleIndex >= 0; sampleIndex--) {
       // We're looping from the end because we'll push some samples to the end
diff --git a/src/test/integration/profiler-edit/__snapshots__/profiler-edit.test.ts.snap b/src/test/integration/profiler-edit/__snapshots__/profiler-edit.test.ts.snap
index 9e919e59d0..8ffda5719f 100644
--- a/src/test/integration/profiler-edit/__snapshots__/profiler-edit.test.ts.snap
+++ b/src/test/integration/profiler-edit/__snapshots__/profiler-edit.test.ts.snap
@@ -87,7 +87,7 @@ Object {
     "markerSchema": Array [],
     "oscpu": "macOS 14.6.1",
     "pausedRanges": Array [],
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "a.out",
     "sampleUnits": Object {
@@ -1452,7 +1452,7 @@ Object {
     "markerSchema": Array [],
     "oscpu": "macOS 14.6.1",
     "pausedRanges": Array [],
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "a.out",
     "sampleUnits": Object {
@@ -2817,7 +2817,7 @@ Object {
     "markerSchema": Array [],
     "oscpu": "macOS 14.6.1",
     "pausedRanges": Array [],
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "a.out",
     "sampleUnits": Object {
@@ -4182,7 +4182,7 @@ Object {
     "markerSchema": Array [],
     "oscpu": "macOS 14.6.1",
     "pausedRanges": Array [],
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "a.out",
     "sampleUnits": Object {
diff --git a/src/test/store/__snapshots__/profile-view.test.ts.snap b/src/test/store/__snapshots__/profile-view.test.ts.snap
index c342a59f5a..0e69b4671b 100644
--- a/src/test/store/__snapshots__/profile-view.test.ts.snap
+++ b/src/test/store/__snapshots__/profile-view.test.ts.snap
@@ -423,7 +423,7 @@ Object {
     "oscpu": "",
     "physicalCPUs": 0,
     "platform": "",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Firefox",
     "sourceURL": "",
@@ -676,7 +676,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -2721,7 +2721,7 @@ CallTree {
         0,
         0,
       ],
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -3109,7 +3109,7 @@ Object {
       0,
       0,
     ],
-    "frame": Array [
+    "frame": Int32Array [
       0,
       1,
       2,
@@ -3563,7 +3563,7 @@ Object {
       0,
       0,
     ],
-    "frame": Array [
+    "frame": Int32Array [
       0,
       1,
       2,
@@ -3949,7 +3949,7 @@ Object {
       0,
       0,
     ],
-    "frame": Array [
+    "frame": Int32Array [
       0,
       1,
       2,
@@ -4335,7 +4335,7 @@ Object {
       0,
       0,
     ],
-    "frame": Array [
+    "frame": Int32Array [
       0,
       1,
       2,
diff --git a/src/test/store/receive-profile.test.ts b/src/test/store/receive-profile.test.ts
index f480f467c9..7a660fa865 100644
--- a/src/test/store/receive-profile.test.ts
+++ b/src/test/store/receive-profile.test.ts
@@ -879,7 +879,10 @@ describe('actions/receive-profile', function () {
       const hash = 'c5e53f9ab6aecef926d4be68c84f2de550e2ac2f';
       const expectedUrl = `https://storage.googleapis.com/profile-store/${hash}`;
 
-      window.fetchMock.get(expectedUrl, _getSimpleProfile());
+      window.fetchMock.get(
+        expectedUrl,
+        serializeProfileToJsonString(_getSimpleProfile())
+      );
 
       const store = blankStore();
       await store.dispatch(retrieveProfileFromStore(hash));
@@ -939,7 +942,7 @@ describe('actions/receive-profile', function () {
       const expectedUrl = `https://storage.googleapis.com/profile-store/${hash}`;
       window.fetchMock
         .getOnce(expectedUrl, 403)
-        .get(expectedUrl, _getSimpleProfile());
+        .get(expectedUrl, serializeProfileToJsonString(_getSimpleProfile()));
 
       const store = blankStore();
       const views = (
@@ -1026,7 +1029,10 @@ describe('actions/receive-profile', function () {
 
     it('can retrieve a profile from the web and save it to state', async function () {
       const expectedUrl = 'https://profiles.club/shared.json';
-      window.fetchMock.get(expectedUrl, _getSimpleProfile());
+      window.fetchMock.get(
+        expectedUrl,
+        serializeProfileToJsonString(_getSimpleProfile())
+      );
 
       const store = blankStore();
       await store.dispatch(retrieveProfileOrZipFromUrl(expectedUrl));
@@ -1063,7 +1069,7 @@ describe('actions/receive-profile', function () {
       // The first call will still be a 403 -- remember, it's the default return value.
       window.fetchMock
         .getOnce(expectedUrl, 403)
-        .get(expectedUrl, _getSimpleProfile());
+        .get(expectedUrl, serializeProfileToJsonString(_getSimpleProfile()));
 
       const store = blankStore();
       const views = (
@@ -1526,7 +1532,9 @@ describe('actions/receive-profile', function () {
           ])
         );
       }
-      window.fetchMock.getOnce('*', profile1).getOnce('*', profile2);
+      window.fetchMock
+        .getOnce('*', serializeProfileToJsonString(profile1))
+        .getOnce('*', serializeProfileToJsonString(profile2));
 
       const { dispatch, getState } = blankStore();
       await dispatch(retrieveProfilesToCompare([url1, url2]));
@@ -1793,7 +1801,9 @@ describe('actions/receive-profile', function () {
     it('gives a fatal error when the selected thread index is out of bounds', async function () {
       const { dispatch, getState } = blankStore();
       const { profile1, profile2 } = getSomeProfiles();
-      window.fetchMock.getOnce('*', profile1).getOnce('*', profile2);
+      window.fetchMock
+        .getOnce('*', serializeProfileToJsonString(profile1))
+        .getOnce('*', serializeProfileToJsonString(profile2));
 
       await dispatch(
         retrieveProfilesToCompare([
@@ -1813,7 +1823,10 @@ describe('actions/receive-profile', function () {
       location: Partial<Location>,
       requiredProfile: number = 1
     ) {
-      const profile = _getSimpleProfile();
+      const simpleProfile = _getSimpleProfile();
+      // Create a profile object we can use with fetchMock, which uses JSON.stringify internally.
+      // `simpleProfile` may contain typed arrays which wouldn't survive JSON.stringfy.
+      const profile = JSON.parse(serializeProfileToJsonString(simpleProfile));
       const geckoProfile = createGeckoProfile();
 
       // Add mock fetch response for the required number of times.
diff --git a/src/test/unit/__snapshots__/profile-conversion.test.ts.snap b/src/test/unit/__snapshots__/profile-conversion.test.ts.snap
index 1ef1824e9b..de79c9b9c5 100644
--- a/src/test/unit/__snapshots__/profile-conversion.test.ts.snap
+++ b/src/test/unit/__snapshots__/profile-conversion.test.ts.snap
@@ -591,7 +591,7 @@ Object {
     "oscpu": undefined,
     "physicalCPUs": undefined,
     "platform": undefined,
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "ART Trace (Android)",
     "sampleUnits": undefined,
@@ -43096,7 +43096,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -83833,7 +83833,7 @@ Object {
     "oscpu": undefined,
     "physicalCPUs": undefined,
     "platform": undefined,
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "ART Trace (Android)",
     "sampleUnits": undefined,
@@ -167262,7 +167262,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -329278,7 +329278,7 @@ Object {
     "oscpu": "",
     "physicalCPUs": 0,
     "platform": "",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Chrome Trace",
     "profilingEndTime": 119159778.026,
@@ -329817,7 +329817,7 @@ Object {
       ],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -364259,7 +364259,7 @@ Object {
     "oscpu": "",
     "physicalCPUs": 0,
     "platform": "",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Chrome Trace",
     "profilingEndTime": 119159778.026,
@@ -364798,7 +364798,7 @@ Object {
       ],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -399219,7 +399219,7 @@ Object {
     "oscpu": "",
     "physicalCPUs": 0,
     "platform": "",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Chrome Trace",
     "profilingEndTime": 66155012.423,
@@ -400087,7 +400087,7 @@ Object {
       ],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -401927,7 +401927,7 @@ Object {
     "oscpu": "",
     "physicalCPUs": 0,
     "platform": "",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Chrome Trace",
     "sourceURL": "",
@@ -401993,7 +401993,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [],
+      "frame": Int32Array [],
       "length": 0,
       "prefix": Array [],
     },
@@ -403765,7 +403765,7 @@ Object {
     "oscpu": "",
     "physicalCPUs": 0,
     "platform": "",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Chrome Trace",
     "profilingEndTime": 355035987.653,
@@ -406722,7 +406722,7 @@ Object {
       ],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -407776,7 +407776,7 @@ Object {
     "oscpu": "",
     "physicalCPUs": 0,
     "platform": "",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Chrome Trace",
     "sourceURL": "",
@@ -411314,7 +411314,7 @@ Object {
       ],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -413399,7 +413399,7 @@ Object {
     "oscpu": "",
     "physicalCPUs": 0,
     "platform": "",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Chrome Trace",
     "profilingEndTime": 66155012.423,
@@ -414267,7 +414267,7 @@ Object {
       ],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -414544,7 +414544,7 @@ Object {
     "oscpu": undefined,
     "physicalCPUs": undefined,
     "platform": undefined,
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Firefox",
     "sampleUnits": undefined,
@@ -417285,7 +417285,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -418704,7 +418704,7 @@ Object {
     "oscpu": undefined,
     "physicalCPUs": undefined,
     "platform": undefined,
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Firefox",
     "sampleUnits": undefined,
@@ -429901,7 +429901,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -434833,7 +434833,7 @@ Object {
     "oscpu": undefined,
     "physicalCPUs": undefined,
     "platform": undefined,
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Firefox",
     "sampleUnits": undefined,
@@ -439076,7 +439076,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -441509,7 +441509,7 @@ Object {
     "oscpu": undefined,
     "physicalCPUs": undefined,
     "platform": undefined,
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Firefox",
     "sampleUnits": undefined,
@@ -454858,7 +454858,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -459410,7 +459410,7 @@ Object {
     "keepProfileThreadOrder": true,
     "markerSchema": Array [],
     "platform": "Android",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "com.example.sampleapplication",
     "sourceCodeIsNotOnSearchfox": true,
@@ -499592,7 +499592,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -521502,7 +521502,7 @@ Object {
     "keepProfileThreadOrder": true,
     "markerSchema": Array [],
     "platform": "Android",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "com.example.sampleapplication",
     "sourceCodeIsNotOnSearchfox": true,
@@ -559132,7 +559132,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -587927,7 +587927,7 @@ Object {
     "oscpu": "",
     "physicalCPUs": 0,
     "platform": "",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "target/debug/examples/work_log (dhat)",
     "sourceURL": "",
@@ -594273,7 +594273,7 @@ Object {
       ],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         56,
         55,
@@ -597177,7 +597177,7 @@ Object {
     "oscpu": "",
     "physicalCPUs": 0,
     "platform": "",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Flamegraph",
     "sourceURL": "",
@@ -600033,7 +600033,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -905382,7 +905382,7 @@ Object {
     "oscpu": "",
     "physicalCPUs": 0,
     "platform": "",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Flamegraph",
     "sourceURL": "",
@@ -905556,7 +905556,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
diff --git a/src/test/unit/__snapshots__/profile-upgrading.test.ts.snap b/src/test/unit/__snapshots__/profile-upgrading.test.ts.snap
index bd079be0b9..3715a6c4fa 100644
--- a/src/test/unit/__snapshots__/profile-upgrading.test.ts.snap
+++ b/src/test/unit/__snapshots__/profile-upgrading.test.ts.snap
@@ -40,7 +40,7 @@ Object {
     "oscpu": undefined,
     "physicalCPUs": undefined,
     "platform": undefined,
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Firefox",
     "sampleUnits": undefined,
@@ -2781,7 +2781,7 @@ Object {
       "startLine": Array [],
     },
     "stackTable": Object {
-      "frame": Array [
+      "frame": Int32Array [
         0,
         1,
         2,
@@ -7643,7 +7643,7 @@ Object {
     "misc": "rv:48.0",
     "oscpu": "Intel Mac OS X 10.11",
     "platform": "Macintosh",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Firefox",
     "stackwalk": 1,
@@ -9019,7 +9019,7 @@ Object {
     "misc": "rv:48.0",
     "oscpu": "Intel Mac OS X 10.11",
     "platform": "Macintosh",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Firefox",
     "stackwalk": 1,
@@ -10563,7 +10563,7 @@ Object {
     "misc": "rv:48.0",
     "oscpu": "Intel Mac OS X 10.11",
     "platform": "Macintosh",
-    "preprocessedProfileVersion": 64,
+    "preprocessedProfileVersion": 65,
     "processType": 0,
     "product": "Firefox",
     "stackwalk": 1,
diff --git a/src/test/unit/json-with-typed-arrays.test.ts b/src/test/unit/json-with-typed-arrays.test.ts
new file mode 100644
index 0000000000..2ba9a2f070
--- /dev/null
+++ b/src/test/unit/json-with-typed-arrays.test.ts
@@ -0,0 +1,129 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import { jsonEncodeObjectWithTypedArraysAsRegularArrays as encode } from '../../utils/json-with-typed-arrays';
+
+describe('jsonEncodeObjectWithTypedArraysAsRegularArrays', function () {
+  it('encodes primitives the same as JSON.stringify', function () {
+    expect(encode(42)).toBe('42');
+    expect(encode('hello')).toBe('"hello"');
+    expect(encode(null)).toBe('null');
+    expect(encode(true)).toBe('true');
+  });
+
+  it('encodes a plain object the same as JSON.stringify', function () {
+    const obj = { a: 1, b: 'two', c: [1, 2, 3], d: { nested: true } };
+    expect(encode(obj)).toBe(JSON.stringify(obj));
+  });
+
+  it('encodes a top-level typed array as a regular array of numbers', function () {
+    const arr = new Int32Array([1, 2, 3, 4]);
+    expect(encode(arr)).toBe('[1,2,3,4]');
+  });
+
+  it('encodes an empty typed array as an empty array', function () {
+    expect(encode(new Uint8Array())).toBe('[]');
+  });
+
+  it('handles all typed array variants', function () {
+    expect(encode(new Int8Array([1, -2]))).toBe('[1,-2]');
+    expect(encode(new Uint8Array([1, 2]))).toBe('[1,2]');
+    expect(encode(new Uint8ClampedArray([1, 2]))).toBe('[1,2]');
+    expect(encode(new Int16Array([1, -2]))).toBe('[1,-2]');
+    expect(encode(new Uint16Array([1, 2]))).toBe('[1,2]');
+    expect(encode(new Int32Array([1, -2]))).toBe('[1,-2]');
+    expect(encode(new Uint32Array([1, 2]))).toBe('[1,2]');
+    expect(encode(new Float32Array([1.5]))).toBe('[1.5]');
+    expect(encode(new Float64Array([1.5]))).toBe('[1.5]');
+  });
+
+  it('encodes typed arrays nested inside an object', function () {
+    const obj = {
+      name: 'data',
+      values: new Int32Array([10, 20, 30]),
+    };
+    expect(encode(obj)).toBe('{"name":"data","values":[10,20,30]}');
+  });
+
+  it('encodes typed arrays nested inside an array', function () {
+    const arr = [1, new Uint8Array([2, 3]), 4];
+    expect(encode(arr)).toBe('[1,[2,3],4]');
+  });
+
+  it('encodes deeply nested typed arrays', function () {
+    const obj = {
+      a: {
+        b: {
+          c: [
+            { d: new Float32Array([1.5, 2.5]) },
+            { e: new Int16Array([7, 8]) },
+          ],
+        },
+      },
+    };
+    expect(encode(obj)).toBe('{"a":{"b":{"c":[{"d":[1.5,2.5]},{"e":[7,8]}]}}}');
+  });
+
+  it('does not mutate the original object', function () {
+    const typedArr = new Int32Array([1, 2, 3]);
+    const inner = { values: typedArr, label: 'x' };
+    const obj = { inner, count: 2 };
+    const originalKeys = Object.keys(obj);
+    const innerKeys = Object.keys(inner);
+
+    encode(obj);
+
+    expect(obj.inner).toBe(inner);
+    expect(inner.values).toBe(typedArr);
+    expect(inner.label).toBe('x');
+    expect(obj.count).toBe(2);
+    expect(Object.keys(obj)).toEqual(originalKeys);
+    expect(Object.keys(inner)).toEqual(innerKeys);
+  });
+
+  it('does not mutate the original array', function () {
+    const typedArr = new Int32Array([1, 2, 3]);
+    const arr = [1, typedArr, 3];
+
+    encode(arr);
+
+    expect(arr[1]).toBe(typedArr);
+    expect(arr).toEqual([1, typedArr, 3]);
+  });
+
+  it('leaves DataView alone (encoded as a regular object)', function () {
+    // DataView is excluded from typed-array handling and falls through
+    // to the regular object path. JSON.stringify on a DataView produces "{}".
+    const buffer = new ArrayBuffer(8);
+    const view = new DataView(buffer);
+    expect(encode(view)).toBe(JSON.stringify(view));
+  });
+
+  it('handles null values inside objects and arrays', function () {
+    const obj = { a: null, b: [null, 1, null], c: new Int32Array([5]) };
+    expect(encode(obj)).toBe('{"a":null,"b":[null,1,null],"c":[5]}');
+  });
+
+  it('handles an empty object and an empty array', function () {
+    expect(encode({})).toBe('{}');
+    expect(encode([])).toBe('[]');
+  });
+
+  it('preserves array element order with typed arrays interleaved', function () {
+    const arr = [
+      new Uint8Array([1]),
+      'middle',
+      new Uint8Array([2]),
+      42,
+      new Uint8Array([3]),
+    ];
+    expect(encode(arr)).toBe('[[1],"middle",[2],42,[3]]');
+  });
+
+  it('encodes the same shared typed array twice when it appears in two places', function () {
+    const shared = new Int32Array([7, 8, 9]);
+    const obj = { first: shared, second: shared };
+    expect(encode(obj)).toBe('{"first":[7,8,9],"second":[7,8,9]}');
+  });
+});
diff --git a/src/types/profile-derived.ts b/src/types/profile-derived.ts
index 75325be75c..89e7cfab68 100644
--- a/src/types/profile-derived.ts
+++ b/src/types/profile-derived.ts
@@ -29,7 +29,6 @@ import type {
   JsTracerTable,
   IndexIntoStackTable,
   WeightType,
-  IndexIntoFrameTable,
   SourceTable,
   IndexIntoSourceTable,
   CounterDisplayConfig,
@@ -200,7 +199,8 @@ export type Counter = {
  * The `StackTable` type of the derived thread.
  *
  * The only difference from the `RawStackTable` is that the `StackTable` has a
- * `category` and a `subcategory` column.
+ * `category` and a `subcategory` column, and that the `StackTable` always uses
+ * a typed array for the `frame` column.
  *
  * The category of a stack node is always non-null and is derived from a stack's
  * frame and its prefix. Frames can have null categories, stacks cannot. If a
@@ -225,8 +225,7 @@ export type Counter = {
  * nsAttrAndChildArray::InsertChildAt stack before transforms are applied.
  */
 export type StackTable = {
-  // Same as in RawStackTable
-  frame: IndexIntoFrameTable[];
+  frame: Int32Array;
   prefix: Array<IndexIntoStackTable | null>;
   length: number;
 
diff --git a/src/types/profile.ts b/src/types/profile.ts
index ad504ac933..fd1ce620f8 100644
--- a/src/types/profile.ts
+++ b/src/types/profile.ts
@@ -60,7 +60,7 @@ export type Pid = string;
  * to storing them as actual lists of frames.
  */
 export type RawStackTable = {
-  frame: IndexIntoFrameTable[];
+  frame: IndexIntoFrameTable[] | Int32Array<ArrayBuffer>;
   prefix: Array<IndexIntoStackTable | null>;
   length: number;
 };
diff --git a/src/utils/json-with-typed-arrays.ts b/src/utils/json-with-typed-arrays.ts
new file mode 100644
index 0000000000..9ab283f35c
--- /dev/null
+++ b/src/utils/json-with-typed-arrays.ts
@@ -0,0 +1,81 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * JSON.stringify an object which may contain typed arrays somewhere in
+ * its structure (nested arbitrarily deeply), with those typed arrays
+ * serialized as regular arrays of numbers.
+ *
+ * Calling JSON.stringify on a typed array would normally give you something
+ * like `{"0": 1, "1": 2}` which is not what you want.
+ *
+ * This function does not mutate rootObject.
+ */
+export function jsonEncodeObjectWithTypedArraysAsRegularArrays(
+  rootObject: unknown
+): string {
+  // We could use JSON.stringify with a "replacer" here.
+  // But instead, we do a full traversal of the object first, possibly
+  // creating new objects (so that the original doesn't get mutated),
+  // and then a regular JSON.stringify with no replacer. This is 5x faster.
+  return JSON.stringify(rewriteTypedArrays(rootObject));
+}
+
+function rewriteTypedArrays(value: unknown): unknown {
+  if (value === null || typeof value !== 'object') {
+    return value;
+  }
+  if (ArrayBuffer.isView(value) && !(value instanceof DataView)) {
+    // Found a typed array! Use Array.from to convert it to a regular
+    // array of numbers.
+    return Array.from(value as unknown as Iterable<number>);
+  }
+  if (Array.isArray(value)) {
+    return rewriteTypedArraysInArray(value);
+  }
+  return rewriteTypedArraysInObject(value as Record<string, unknown>);
+}
+
+function rewriteTypedArraysInArray(arr: readonly unknown[]): unknown[] {
+  let result: unknown[] | null = null;
+  for (let i = 0; i < arr.length; i++) {
+    const el = arr[i];
+    // Inline fast-path for primitives: an array of 1000 numbers walks this
+    // loop without any function call.
+    if (el === null || typeof el !== 'object') {
+      continue;
+    }
+    const replaced = rewriteTypedArrays(el);
+    if (replaced !== el) {
+      if (result === null) {
+        result = arr.slice();
+      }
+      result[i] = replaced;
+    }
+  }
+  return result ?? (arr as unknown[]);
+}
+
+function rewriteTypedArraysInObject(
+  obj: Record<string, unknown>
+): Record<string, unknown> {
+  let result: Record<string, unknown> | null = null;
+  for (const key in obj) {
+    if (!Object.hasOwn(obj, key)) {
+      continue;
+    }
+    const el = obj[key];
+    if (el === null || typeof el !== 'object') {
+      continue;
+    }
+    const replaced = rewriteTypedArrays(el);
+    if (replaced !== el) {
+      if (result === null) {
+        result = { ...obj };
+      }
+      result[key] = replaced;
+    }
+  }
+  return result ?? obj;
+}