Below is the common error that I always have when I force my Firefox to close.
Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.

You can’t fix it by log out from the system. You can only do 2 things, kill the Firefox process or restart your system. The second option will take a longer time, so let’s do the first one.
Open your Linux terminal (Applications>Accessories>Terminal) then type: ps ax | grep firefox.
You will see the PID number on the left side. PID is Program Identifier. The number can be different.
Have to have stop the process by kill the PID one by one.

Now, start your Firefox again. If you see the screen below mean the problem is fixed.

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