If your Firefox does not open, it's probably broken because of the corrupted files. In this case, go to Start > All Programs > Mozilla Firefox > Mozilla Firefox (Safe Mode). It's possible the error is caused by extensions. Try to disable all the extensions and then start it again. If it starts, enable one extension at the time and restart the browser then do this for all the extensions until it doesn't start again.
You can use this trick to see what is causing the error. Alternatively, just re-install the application from its official website.