May be your profile is corrupted. It does happen sometimes when firefox
abends. It could also be an addon that is in problem. In that case you have to
disable all addons and bring them back one by one until you find the problem.

Try to start firefox in the profile manager first and see if you can create a
new profile that works.

You have to start it from a shell with: firefox -ProfileManager&

If it does work with a new profile, then copy your bookmarks/cookies from the
old profile, delete it an set the new one as the default.