firefox: desktop profile user.js only while its Firefox runs

The desktop profile's user.js link (undoing the fullscreen fix there) was
kept in place on every switch and before every launch. Now the launcher
(nested desktop only) makes it right before Firefox starts, waits for
Firefox instead of exec'ing it, and once it has exited and the profile is
no longer in use removes the link and the value Firefox stored from it in
prefs.js. A second launch that hands its URL to the running Firefox returns
at once and leaves both alone; a user.js of the user's own is never
touched. The desktop profile itself is a normal Firefox profile (browser
data) and is never removed.

No activation step touches profiles anymore: leftovers of a crash and what
older versions wrote (user.js copies and links, their prefs.js values) are
steam-frame-nix-cleanup's (orphans on switch), which leaves the desktop
link alone while the profile is in use; the firefox-desktop-userjs --keep
is gone. The launcher lives in firefox/launcher.nix; checks.firefox runs it
against a fake flatpak (link only while running, forwarded second launch,
prefs.js cleaned only once unlocked, own user.js untouched, Steam session).

flake.nix

@@ -56,10 +56,11 @@
     }) self.packages;
 
     # Tests of the VR keyboard (text model, corrector, swipe decoder on the
-    # default German + English dictionary), the Jellyfin mpv shim and
-    # install.sh cleanup: nix flake check
+    # default German + English dictionary), the Jellyfin mpv shim, the
+    # Firefox launcher and install.sh cleanup: nix flake check
     checks = forSystems (pkgs: {
       cleanup = import ./modules/cleanup/check.nix { inherit pkgs; };
+      firefox = import ./modules/firefox/check.nix { inherit pkgs; };
       jellyfin = import ./modules/jellyfin/check.nix { inherit pkgs; };
       vr-keyboard = (import ./modules/vr-keyboard/build.nix {
         inherit pkgs;