Below is the common error that I always have when I force my Firefox to close.
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.