Acording to this

… but here’s a tip: prefs or other JavaScript that you don’t want overwritten (e.g. comments) can be put in a file called user.js …

Mozilla has what I’d call a user init script, aka a .rc file. But Firefox’s doc doesn’t say that (see here) and my experiments suggest that the only thing can put in a firefox user.js file is calls on user_pref. Everything else is ignored? Bummer.