Hello,
with CentOS 4.3 (i386), after updating firefox to 1.5 from update repo, I can't get it to have menu in my native language (french), altough french language pack appears in extension list. I've tried many things, like moving it on top/down, disabling/removing other languages packs, creating a fresh new user, and reinstalling firefox, to no avail (yes, I restarted firefox between these attempts). I also searched in upstream's bugzilla, and didn't find any issue for v4 & firefox...
Finally, launching firefox through strace, I find that it searches for /usr/lib/firefox-1.5.0.5/chrome/fr-FR.jar and can't find it, since french lang pack seems to be /usr/lib/firefox-1.5.0.5/extensions/langpack-fr@firefox.mozilla.org/chrome/fr.jar
I filed the issue in CentOS bugtracker: http://bugs.centos.org/view.php?id=1442
PS: if that matters, I'm using KDE from kde-redhat repo...