Firefox writes ~/.config/mozilla/firefox/profiles.ini itself on first launch,
pointing at an auto-created random-prefix profile. Home-manager then refuses to
clobber that file and fails the whole activation, so the declarative default
profile — search.json.mozlz4 (DuckDuckGo) and the Stylix user.js — never lands
and Firefox keeps running the stale profile with Google as default and no theme.
Set force = true on the generated profiles.ini home.file entry so home-manager
owns it, deploys the default profile, and Firefox switches to it.