In the event something goes wrong with a new extension or theme you installed, you can start Firefox in Safe Mode. Safe Mode does not load any extensions and comes up with the default theme. This way you can uninstall the offending extension or theme and get things back to normal. To run Firefox safe mode, click on the Start button and select 'Run'. Copy this line:
"C:\Program Files\Mozilla Firefox\firefox.exe" -safe-mode
You can now go to the 'Tools->Add-ons' menu option and uninstall any offending extension or theme. Once this is done, close Firefox then restart it and everything should be good to go.