If you are having problems with Firefox due to an add-on or modification, you can correct the problem in Safe Mode.
By default an icon is installed called Mozilla Firefox (Safe Mode) that you can use.
If this icon is not available, you can do the following :
In Windows go to Start > Run and type [sourcecode language='html']firefox -safe-mode[/sourcecode]
In Mac OS X open Applications > Utilities > Terminal > Run (for Firefox): and type [sourcecode language='html']/Applications/Firefox.app/Contents/MacOS/firefox -safe-mode[/sourcecode]