See: *"Hang at exit": http://kb.mozillazine.org/Firefox_hangs *"Firefox hangs when you quit it": https://support.mozilla.org/kb/Firefox+hangs Make sure that Firefox closes properly and that there isn't a hanging Firefox or plugin-container process left in the task manager. Try to use "Firefox/File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit") to close Firefox if you are currently doing that by clicking the close X on the title bar. You can also check for issues with plugins. *https://support.mozilla.org/kb/Troubleshooting+plugins