How long does that go on? Firefox needs a variable amount of time to "clean up" at shutdown. For example, if Firefox is set to clear history when it closes, then it will process that instruction after the window leaves the screen for up to 60 seconds. I think after 60 seconds if it hasn't finished, it will show a crash dialog. If you are seeing something else -- basically Firefox running by itself -- it could indicate a malware problem. See: [[Troubleshoot Firefox issues caused by malware]].