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Firefox thinks there's another instance running, but not true

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I cannot use, update, uninstall, refresh, or reinstall Firefox (and haven't for four or more years). I always get this message: "Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window." But I have (obviously) shut down the computer entirely any number of times in all those years. What can I do? I will be taking college classes and Firefox is one of the few browsers that the college supports.

I cannot use, update, uninstall, refresh, or reinstall Firefox (and haven't for four or more years). I always get this message: "Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window." But I have (obviously) shut down the computer entirely any number of times in all those years. What can I do? I will be taking college classes and Firefox is one of the few browsers that the college supports.

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Asịsa Ahọpụtara

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Thank you so much. It was the parent.lock file. I was able to refresh Firefox. Just took a few moments.