I think it might make sense to reinstall Firefox over itself to resolve any problems with the program files. This shouldn't affect your personal settings, but just in case, you should back up your profile. To back up Firefox, see [https://support.mozilla.com/en-US/kb/Backing+up+your+information Backing up your information]. If you want to try something else first, you could use Firefox's Safe Mode to see whether disabling add-ons has any effect.[http://support.mozilla.com/kb/Safe+Mode Safe Mode]. Be careful not to "reset" anything permanently if you didn't back up.