When quitting a pogram
When you cancel a program, you cannot start another one because Firefox is not responding and first the"old" Firefox must be stopped. This is onerous. Why not automatically (i.e. with a software delete) stop the "old" Firefox each time a Firefox-supported program is cancelled?
Wšykne wótegrona (1)
This situation sounds like a corrupted Firefox browser if you can't just close with the "x" or exit selections then you got a corrupted Browser. You need to find out if this is Addon or corrupted Browser by using Firefox in Private Mode Browser and see if that can be closed or not to verify if there is Addon issue or corrupted Browser.