Is the Back Button Greyed Out in Firefox Safe Mode?
Before checking which of your extensions is responsible for this error, you should first run Firefox in Safe Mode in order to see if the back button is still greyed out. To do this, go to Start > Programs > Mozilla Firefox > Mozilla Firefox (Safe Mode) – this will open the application in Safe Mode, and you should start using it straight away to find out if the back button is greyed out. Use any page upon which this fault has previously occurred, and also try it with multiple tabs open.