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).

install.sh

@@ -74,8 +74,7 @@ Commands:
                      or before removing it.
         --orphans    what the configuration no longer uses (run by the Home
                      Manager module on every switch); keeps saved choices.
-        --keep       still in use (with --orphans): debugger,
-                     firefox-desktop-userjs=<profile>
+        --keep       still in use (with --orphans): debugger
         --dry-run    only print what would be done
         --quiet      print only actions, deferrals and warnings
       SteamVR's VRWebHelper.DebuggerEnabled can't be changed while SteamVR
@@ -396,10 +395,12 @@ EOF
 #   icons      links hicolor/scalable/apps/<name>.svg -> Breeze in the store
 #              (the icon-fallbacks script of 2026-09; manifest
 #              $SFN_STATE/icon-fallbacks).
-#   firefox    user.js in Firefox profiles: links to the desktop profile's
-#              /app/etc/firefox/steam-frame-nix-desktop-user.js, older links
-#              to *-firefox-*user.js and copies starting with FF_MARKER, and
-#              the values they left in prefs.js (only with Firefox closed).
+#   firefox    user.js in Firefox profiles: links to
+#              /app/etc/firefox/steam-frame-nix-desktop-user.js (the
+#              launcher's, while the desktop profile runs; left alone while
+#              the profile is in use), older links to *-firefox-*user.js and
+#              copies starting with FF_MARKER, and the values they left in
+#              prefs.js (only with Firefox closed).
 #   jellyfin   the hwdec shim entries in the Jellyfin Flatpak's user override
 #              (nix-flatpak), an empty override file, and the shim copy of
 #              earlier versions (marker $SFN_STATE/jellyfin-hwdec-shim).
@@ -714,8 +715,8 @@ clean_icons() {
 ff_keys() { sed -n 's/^[[:space:]]*user_pref(\("[^"]*"\),.*/\1/p' "$1" 2>/dev/null || true; }
 ff_in_use() { find /proc/[0-9]*/fd -lname "$1/.parentlock" -print -quit 2>/dev/null | grep -q .; }
 
-clean_firefox() { # keep_profile ('' = none)
-  local keep=$1 prof name u t kind keys pats left
+clean_firefox() { # all
+  local all=$1 prof name u t kind keys pats left
   [[ -d $FF_DIR ]] || return 0
   for prof in "$FF_DIR"/*/; do
     prof=${prof%/}; name=${prof##*/}; u=$prof/user.js
@@ -725,7 +726,11 @@ clean_firefox() { # keep_profile ('' = none)
       t="$(readlink "$u")"
       case $t in
         "$FF_DESKTOP_JS")
-          [[ $name == "$keep" ]] && continue
+          # The launcher's, while Firefox runs in the desktop profile.
+          if ff_in_use "$prof"; then
+            (( all )) && c_defer "$u: Firefox is using profile $name; close it and run this again"
+            continue
+          fi
           kind="desktop profile user.js link"; keys='"full-screen-api.ignore-widgets"' ;;
         /nix/store/*-firefox-user.js|/nix/store/*-firefox-desktop-user.js)
           kind="user.js link of an older version"; keys="$(ff_keys "$u")"
@@ -846,7 +851,7 @@ clean_ui_state() { # all
 }
 
 cmd_cleanup() {
-  local mode='' keep_debugger=0 keep_ff='' keeps=0 k
+  local mode='' keep_debugger=0 keeps=0 k
   while (( $# )); do
     case $1 in
       --all) mode=all; shift ;;
@@ -858,8 +863,7 @@ cmd_cleanup() {
         k=$2; shift 2; keeps=$((keeps + 1))
         case $k in
           debugger) keep_debugger=1 ;;
-          firefox-desktop-userjs=?*) keep_ff=${k#*=} ;;
-          *) die "cleanup: unknown artifact '$k' for --keep (debugger, firefox-desktop-userjs=<profile>)" ;;
+          *) die "cleanup: unknown artifact '$k' for --keep (debugger)" ;;
         esac ;;
       -h|--help) usage; exit 0 ;;
       *) die "cleanup: unknown option '$1' (see --help)" ;;
@@ -875,7 +879,7 @@ cmd_cleanup() {
   if (( ! CLEAN_QUIET )); then step "$CLEAN_HEADER"; CLEAN_HEADER=''; fi
   clean_debugger "$keep_debugger"
   clean_icons
-  clean_firefox "$keep_ff"
+  clean_firefox "$([[ $mode == all ]] && echo 1 || echo 0)"
   clean_jellyfin
   clean_ui_state "$([[ $mode == all ]] && echo 1 || echo 0)"
   c_rmdir "$SFN_STATE"