Is there something in the prefs.js (home dir) to change that? I've noticed since they changed the preferences dialog, it's missing all kinds of "important" stuff, ability to shut off (in that dialog) the auto image-resizing, change control of tabbed browsing, for example(s).
[EDIT]
I just realized, you're probably talking more about the interface than the http accept languages, which, I'm not sure if the Mozilla language packs are drop-in compatible with Firebird or not.
If you are talking about the http accept languages, as you probably already know, you can try putting:
user_pref("intl.accept_languages", "en-us, en");
into your prefs.js file, changing en-us,en to whatever you like.
[/EDIT]