Você pode tentar redefinir todas as preferências do Firefox para as configurações padrão. De acordo com a página de suporte do Firefox :
At the top of the Firefox window, click the Firefox button, go over to the
Help
menu and selectRestart with Add-ons Disabled...
Firefox will start up with the Firefox Safe Mode dialog.For Windows XP, click the
Help
menu and selectRestart with Add-ons Disabled...
1: To reset your user preferences, click to put a check mark by
Reset all user preferences to Firefox defaults
.2: To apply your changes, click Make Changes and Restart.
Firefox will restart with your settings changed back to the defaults.