Exit Firefox completely
Start Firefox in Safe Mode. The "Firefox Safe Mode" window will appear with some troubleshooting options
Select the option, Reset all user preferences to Firefox defaults.
Click the button, Make Changes and Restart.
Try dat.
Starting with Firefox 4, SeaMonkey 2.1, and Thunderbird 3.3, you can restart in Safe Mode via "Help -> Restart with Add-ons disabled...". When Firefox or SeaMonkey restarts, a Safe Mode options window will appear (see below)