So it does literally nothing?
Is it the only button that doesn't work?
Run Firefox in safe-mode ( Firefox safe-mode not Windows - you'll have a shortcut to it in your Start menu ). If the problem stops then go back to normal firefox and try disabling addons one by one ( starting with the most recently added or updated ).
I'm not sure it's addon related since we don't have much information but with Firefox it's always a good place to start.