Occasionally you may get an error when attempting to start the Firefox web browser.
Firefox is already running. Close or restart computer.
This error is usually due to a Firefox process that was not properly terminated the last time Firefox was closed.
This problem can be remedied by using ls -la | grep firefox at the command line to determine the PID of the running process and then using the kill command to terminate it.
Another option is to logout of the desktop GUI session and login again. Since Firefox is run as the user of the login GUI session, it will be killed along with all of the other programs started by the user of that login session.