firefox: prefs as default prefs through the systemconfig extension

prefs and vrFullscreenFix become default prefs (pref()) in
defaults/pref/steam-frame-nix.js of /app/etc/firefox, the mount point of
the org.mozilla.firefox.systemconfig extension point. home-manager
provides the extension as a user "unmaintained extension" link
($XDG_DATA_HOME/flatpak/extension/<id>/aarch64/stable) to a store dir,
which Flatpak mounts itself: the sandbox doesn't see /nix. Default prefs
are never written to prefs.js, so removing one leaves nothing behind.

The desktop profile undoes the fullscreen fix with a user.js linked to
/app/etc/firefox/steam-frame-nix-desktop-user.js. The sync (on switch,
and before each launch) removes the user.js copies and store links of
older versions and takes their values out of prefs.js, for each profile
not in use (Firefox holds .parentlock open).

README.md

@@ -257,7 +257,8 @@ without it.
 | `steamFrame.clipboardSync.enable` | bool | `true` | Clipboard bridge between the Steam session and the nested desktop. |
 | `steamFrame.clipboardSync.package` | package | built from `dnut/clipboard-sync` | The clipboard-sync package. |
 | `steamFrame.firefox.enable` | bool | `false` | Launcher for the Flathub Firefox Flatpak with the fixes below. |
-| `steamFrame.firefox.vrFullscreenFix` | bool | `true` | Link a `user.js` with `full-screen-api.ignore-widgets` into profiles. |
+| `steamFrame.firefox.vrFullscreenFix` | bool | `true` | Default `full-screen-api.ignore-widgets` to `true` (not in the desktop profile). |
+| `steamFrame.firefox.prefs` | attrs of bool, int or str | `{ }` | `about:config` default values for every profile, e.g. `{ "media.av1.enabled" = false; }`. |
 | `steamFrame.firefox.desktopProfile` | null or str | `"desktop"` | Separate profile for the nested desktop; `null`: none. |
 | `steamFrame.jellyfin.hardwareDecoding.enable` | bool | `false` | Hardware video decoding in the Jellyfin Desktop Flatpak, see [Jellyfin](#jellyfin-hardware-decoding-jellyfinhardwaredecoding). |
 | `steamFrame.jellyfin.hardwareDecoding.hwdec` | str | `"v4l2m2m-copy,auto-copy"` | mpv `hwdec` used instead of Jellyfin's automatic one. |
@@ -807,14 +808,35 @@ launcher shadows the Flatpak's own entry (same ID), so default-browser
 associations keep working.
 
 - **`vrFullscreenFix`:** gamescope never shows fullscreen windows, so
-  Firefox looks frozen. A `user.js` with `full-screen-api.ignore-widgets`
-  makes fullscreen fill just the window; it is linked into every profile
-  (existing non-symlink `user.js` files are left alone). **Remove when**
-  gamescope shows fullscreen X11 windows in VR.
+  Firefox looks frozen. `full-screen-api.ignore-widgets` makes fullscreen
+  fill just the window. **Remove when** gamescope shows fullscreen X11
+  windows in VR.
+- **`prefs`:** `about:config` values for every profile. E.g.
+  `"media.av1.enabled" = false`: the Frame's decoder has no AV1, so YouTube
+  and co. fall back to VP9/H.264, which it decodes in hardware.
 - **`desktopProfile`:** the sessions can't see each other's Firefox, so a
   second instance stops at the locked profile; in the nested desktop the
   launcher uses this separate profile.
 
+`prefs` and the fullscreen fix are *default* values (`pref()`), not user
+values: `about:config` can still change them per profile, and nothing is
+written to `prefs.js`, so removing one leaves nothing behind. Firefox reads
+default prefs from `defaults/pref/*.js` in its system config dir, which in
+the Flatpak is `/app/etc/firefox`, the mount point of the
+`org.mozilla.firefox.systemconfig` extension. home-manager provides that
+extension as a link from
+`~/.local/share/flatpak/extension/org.mozilla.firefox.systemconfig/aarch64/stable`
+to a store directory; Flatpak mounts it itself, so the sandbox doesn't get
+`/nix`. (A systemconfig extension of your own would conflict with it.) The
+desktop profile undoes the fullscreen fix with a `user.js` linked to
+`/app/etc/firefox/steam-frame-nix-desktop-user.js` (a sandbox path).
+Changes take effect at the next start of Firefox.
+
+Older versions copied a `user.js` into every profile; the copies and the
+values they left in `prefs.js` are removed on switch and before each launch,
+for each profile not in use at that moment. `user.js` files of your own are
+left alone.
+
 ### Jellyfin hardware decoding (`jellyfin.hardwareDecoding.*`)
 
 For the Flathub [Jellyfin Desktop](https://github.com/jellyfin/jellyfin-desktop)