If Firefox no longer starts up after you installed an add-on, what you can do is start Firefox in safe mode and just disable that add on.
To do this, open up a console and run the following:
You’ll see the following dialog box:
Press the Continue in Safe Mode button. Once Firefox has started up, go to the Tools->Add-ons menu and disable the one you just installed. Click the Restart Firefox button, and it should start up fine again.