A day after presenting the question, seems like a user got a problem I didn’t thought of. He didn’t like Firefox 3 and decided to install Firefox 2 instead.
The problem is that he now has Firefox 2 and the translation for Firefox 3 (from the GNOME package). Same problem occurs with the Hebrew interface… The solution is to manually delete the Firefox 3 files, and than install the mozilla-firefox-locale-he-il package.
Don’t forget that this should be done on every update of the language-pack-gnome-he-base package (or any other language than Hebrew)! So who’s in favor of splitting the Firefox 3 translation to another package ?