This is the error message that tells you Firefox is already running:
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.
To fix it you can usually just kill Firefox with:
But sometimes it decides to be a bit of a pest and you’ll need to find the Firefox PID and then kill the process:
ps auxwww | grep firefox
Which will give you something like this:
rich 2167 0.0 0.0 3300 764 pts/0 S+ 19:36 0:00 grep firefox
Then you just kill the process number:
Still not able to kill Firefox?
1. Find your profile. This page tells you how to find the location of your Firefox profile. Under Linux (e.g. Ubuntu), it will be at ~/.mozilla/firefox/[Profile name]/ .
2. Remove the lock files. This page tells you what the lock files are for Firefox on Windows/Linux/Mac. Under Unix/Linux, you’ll need to remove two files “lock” and “.parentlock” .