From Fedora Project Wiki
Standard troubleshooting steps
Test in Safe Mode ( firefox -safe-mode )
Reset preferences ( Safe mode option )
Troubleshoot plugins Disable plugins ( Ex: Adobe Reader, Flash, Java, QuickTime, RealPlayer, and VLC )
Make a new profile ( firefox -ProfileManager )
Slowness when browsing?
Go to your profile folder and vacuum your db files ( cd $HOME/.mozilla/firefox/*.default/; for i in *.sqlite; do echo "VACUUM;" | sqlite3 $i ; done )
Disable favicon and site-icon caching
Turn off IPv6