Recent Gray Richmond's article on Free Software Magazine "How to fix broken Firefox extensions" covered this issue extensively. The article talks about getting dirty with XPI files (a.k.a extension files) but not everyone is familiar with XUL so disabling a troublesome extension/theme/custom setting using safe mode is the best way out.
If you are still having problems, your best bet is to start Firefox in what is called Safe Mode. To do this you will need to fire up a console and type firefox -safe-mode. Firefox should now start but all extensions will now be disabled, including any offending ones which caused the problem in the first place. It is simply a matter of disabling/removing the relevant extensions, closing Firefox and restarting it in the normal manner. You can find out more about using Safe Mode from Mozillazine.
Lets see the safe mode in action here. First get rid of any existing process of the browser with killall command. And then start the browser with --safe-mode option on the console (Firefox users use "firefox-bin" instead of "iceweasel").
killall -11 firefox-bin iceweasel --safe-mode
The browser will start in the safe mode with a dialog box which temporarily allows you to disable the offending extension/theme/custom setting. Either you can choose to continue loading the browser in safe mode and disable the extension from Tools > Add-ons manager or just disable everything and restart the browser in normal mode.