How do you fix the error "Firefox is already running but not responding"?

•  Firefox users have been plagued by the problem 'Firefox is already operating but not responding.' When you get this issue, you don't need to restart your computer; you can generally fix it by immediately heading to Task Manager.
• The nuclear option is to restart your computer, which, frankly, isn't a good idea. You can still fix this error without restarting your computer if you encounter it.
• If you've tried everything above, including restarting your computer, and you're still getting this message, it's possible that your Firefox profile is 'locked.'
• Firefox is designed to ensure that a profile is only used by one copy of Firefox at a time. Special lock files are placed in the profile directory to ensure this. When Firefox crashes or closes abruptly, it may leave the file locked in unusual instances.

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