fix: Add debug info when an invalid folder is trying to restore, b=no-bug, c=common, folders

Author: mr-cheffy

prefs/view.yaml

@@ -51,3 +51,6 @@
 
 - name: zen.view.context-menu.refresh
   value: false
+
+- name: zen.view.borderless-fullscreen
+  value: true

src/zen/common/zenThemeModifier.js

@@ -96,7 +96,8 @@ var ZenThemeModifier = {
     if (
       window.fullScreen &&
       window.gZenCompactModeManager?.preference &&
-      !document.getElementById('tabbrowser-tabbox')?.hasAttribute('zen-split-view')
+      !document.getElementById('tabbrowser-tabbox')?.hasAttribute('zen-split-view') &&
+      Services.prefs.getBoolPref('zen.view.borderless-fullscreen', true)
     ) {
       separation = 0;
     }

src/zen/folders/ZenFolders.mjs

@@ -1615,18 +1615,25 @@
           const parentWorkingData = tabFolderWorkingData.get(stateData.parentId);
           if (parentWorkingData && parentWorkingData.node) {
             switch (stateData?.prevSiblingInfo?.type) {
-              case 'group': {
-                const folder = document.getElementById(stateData.prevSiblingInfo.id);
-                folder.after(node);
-                break;
-              }
               case 'tab': {
                 const tab = parentWorkingData.node.querySelector(
                   `[zen-pin-id="${stateData.prevSiblingInfo.id}"]`
                 );
                 tab.after(node);
                 break;
               }
+              case 'group': {
+                const folder = document.getElementById(stateData.prevSiblingInfo.id);
+                if (folder) {
+                  folder.after(node);
+                  break;
+                }
+                // If we didn't find the group, we should debug it and continue to default case.
+                console.warn(
+                  `Zen Folders: Could not find previous sibling group with id ${stateData.prevSiblingInfo.id} while restoring session.`
+                );
+                // @eslint-disable-next-line no-fallthrough
+              }
               default: {
                 // Should insert after zen-empty-tab
                 const start =