I had a trouble with Mozilla too. After installing russian language support Mozilla wouldn't launch at all. It says somethig about not being able to find (or set?) XBL bindings.... I could still run it as root, but I don't know too many people who would browse the internet as root.
I got it working again by restoring settings to defaults, but the problem remains.
Though I don't really care whether the programm interface is in english or in russian, some of our customers would certainly prefer native language over default. It turns to be FC2 specific, because nothing like it had ever happened with other distros....
Any ideas what could be done to fix this bug?