fix(browsers): switch to the German pack, and off what the first run showed

languagePacks installs the pack and the policy reports it as active, but the
interface stayed English: installing is not switching, the same split the theme
already had. Firefox does not fall back to LANG either, although de_DE.UTF-8
reaches the process. RequestedLocales is what switches to it.

Three settings from the same first run: address autofill, payment autofill, and
the three Firefox Suggest switches. The autofill pair are preferences. Suggest
is a policy instead, because its preferences already ship as false and the
profile sets no user value while the switches were on — something enables them
at runtime, which a preference set as a default does not survive. ImproveSuggest
is named rather than left to follow from the other two, so the data collection
cannot come back if Suggest is ever turned on again.

DECISIONS.md

@@ -70,6 +70,21 @@
   `let` binding because both read it. Verified at eval that the module's own
   ExtensionSettings entry for the German language pack survives alongside the two
   added ones — the option is a JSON-format type, so attribute sets merge per key.
+  The language pack turned out to have the very same split, which only showed on
+  the machine: `languagePacks` installs it and the policy reports it as active,
+  but the interface stayed English, and Firefox does not fall back to `LANG`
+  either. `RequestedLocales = [ "de" ]` is what switches to it.
+
+  The first run on the machine turned up three more, all of them settings the
+  browser presents as switches: address autofill, payment autofill, and the
+  three Firefox Suggest switches. The autofill pair are plain preferences.
+  Suggest needed the `FirefoxSuggest` policy instead, for a reason worth
+  keeping: its preferences already ship as `false` and the profile sets no user
+  value, yet the switches were on — something turns them on at runtime, and a
+  preference set as a default does not hold against that. `ImproveSuggest`, the
+  data collection behind Suggest, is stated rather than left to follow from the
+  other two, so it cannot return if Suggest is ever switched back on.
+
   Two of the settings are starting points rather than fixed states, which the
   policies cannot change: DisplayBookmarksToolbar sets the toolbar's initial
   state and the View menu still brings it back, and Homepage without `Locked`

modules/browsers.nix

@@ -38,6 +38,13 @@ in
       # this, Firefox starts on the default look with Catppuccin sitting unused
       # in the add-on manager.
       "extensions.activeThemeID" = catppuccinThemeId;
+
+      # The two autofill switches in the privacy settings, "Save and fill
+      # addresses" and "Save and fill payment methods". Each carries a capture
+      # half as a child preference, so one switch covers both storing and
+      # filling. Betterfox leaves these alone; Firefox ships them on.
+      "extensions.formautofill.addresses.enabled" = false;
+      "extensions.formautofill.creditCards.enabled" = false;
     };
 
     # normal_installed rather than force_installed: both add-ons stay removable.
@@ -57,6 +64,27 @@ in
         };
       };
 
+      # languagePacks installs the German pack, it does not switch to it — the
+      # same split as the theme above. Firefox does not derive this from LANG
+      # either: without the policy the interface stays English with the pack
+      # sitting installed and active.
+      RequestedLocales = [ "de" ];
+
+      # The three suggestion switches in the search settings. Their preferences
+      # ship as false and no user value sets them, so something enables them at
+      # runtime — the policy is what holds against that. From Firefox 146 on,
+      # WebSuggestions switches Suggest off as a whole; this is 156.
+      #
+      # ImproveSuggest is the data collection behind it: what gets typed into
+      # the address bar goes to Mozilla to improve the suggestions. It is stated
+      # rather than left to follow from the other two, so it does not come back
+      # if Suggest is ever turned on again.
+      FirefoxSuggest = {
+        WebSuggestions = false;
+        SponsoredSuggestions = false;
+        ImproveSuggest = false;
+      };
+
       SearchEngines.Default = "DuckDuckGo";
       OfferToSaveLogins = false;