mkFirefoxModule: document impermanence extensions

Clarify that extensions.autoDisableScopes only handles manual extension approval. Firefox still owns mutable extension runtime state, so impermanent setups must persist the relevant profile state or configure supported extension settings declaratively.

Closes nix-community#6398

Author: khaneliman

modules/programs/firefox/mkFirefoxModule.nix

@@ -692,6 +692,20 @@ in
                         `"extensions.autoDisableScopes" = 0;`
                         to
                         [{option}`${moduleName}.profiles.<profile>.settings`](#opt-${moduleName}.profiles._name_.settings)
+
+                        On systems using impermanence, this only prevents
+                        ${name} from requiring manual extension approval. It
+                        does not preserve extension runtime state such as
+                        extension UUIDs, logins, local storage, or
+                        per-extension data. Persist the ${name} profile state
+                        needed by your extensions, or configure supported
+                        extension settings declaratively with
+                        [{option}`${moduleName}.profiles.<profile>.extensions.settings`](#opt-${moduleName}.profiles._name_.extensions.settings).
+
+                        Persisting only the `extensions` directory is generally
+                        not sufficient, because ${name} stores extension state
+                        in other profile files and databases that are managed
+                        outside Home Manager.
                       '';
                     };