See this - https://support.mozilla.org/en-US/kb/firefox-already-running-not-responding#w_end-firefox-processes

gelnd said

Sometimes when I open Firefox I get a box (shouldn't have used the word POPUP--it distracted the search function)--anyhow, a box saying Firefox already running and I select the CLOSE FIREFOX button and everything is fine but today CLOSE FIREFOX refuses to close firefox.

Is there any place to say thank you Thank You THANK YOU? If there isn't there should be. i am 85 years old and have never been technically proficient, but your instructions worked so thank you Thank You THANK YOU.

Hello,

I am glad to hear that your problem has been resolved. If you haven't already, please select the answer that solves the problem. This will help other users with similar problems find the solution more easily.

Thank you for contacting Mozilla Support.