Safe Mode is a special Firefox mode that can be used to troubleshoot and fix problems. Safe Mode temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes) that might be causing problems. By comparing Firefox behavior in normal mode to its behavior in Safe Mode you may be able to pinpoint the cause of the problem.
Click the menu button , click help and select Restart with Add-ons Disabled.... Firefox will start up with the Firefox Safe Mode dialog.
You now have two options:
Once Firefox is in Safe Mode, you'll want to test its behavior and see if the problem goes away.
If the problem still happens in Safe Mode, it is not being caused by an extension or theme. Other possible causes could be plugins or changes made to Firefox preference settings, which are not disabled in Safe Mode. See the following articles for solution:
If the problem does not happen in Safe Mode, it is most likely because of an extension, theme or hardware acceleration.
Based on information from Safe mode (mozillaZine KB)