Hello, Open the Task manager>> You can see Firefox running there try to close it, in case it dont get closed>> Click on Processes Tab in the Task manager>> Look for Firefox Process running out there >> Stop (Kill or end) the process from there.

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When I open the Task manager, it does not list Firefox as running. When I switch to the processes tab, Firefox is listed as a running process. When I try to close it there, it will not close.

See this: Kill Programs

Reply back to us and let us know if that helped you out in any manner.

Did you also look for plugin-container processes that might prevent Firefox from getting closed?

Use "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: Firefox > Quit Firefox; Linux: Firefox/File > Quit) to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

See also:

• "Hang at exit": http://kb.mozillazine.org/Firefox_hangs
• "Firefox hangs when you quit it": https://support.mozilla.org/kb/Firefox+hangs

From my point of view Firefox is not open. However if I try to open it, the system says it is already open. Therefore I cannot exit Firefox.

I cannot understand your description. Since the problem is rare and intermittent, I can't test any suggestions until the next time it happens.

Using "Kill" looks complicated. Is it any better than "restart", which has been my solution?

No, Killing a process is not as complicated as it seems, just follow the steps as shown in the article. Killing means forcefully closing the Process and forcing it to release all the resources it is using.