Firefox is already running, but is not responding

Sometimes when you try to open Firefox you will get the error message "Firefox is already running, but is not responding". This most commonly happens when Firefox crashes or fails to shut down properly e.g. because of a power cut. It can also be caused by attempting to run Firefox on two Linux computers at the same time (in which case, just close one of the browsers down).

First kill any stuck Firefox processes.

killall firefox
killall firefox-bin
killall -9 firefox
killall -9 firefox-bin
ps uxwww | grep "[f]irefox"

The final command should give no output. If any stubborn firefox processes have survived, reboot the computer before going on to the next stage.

If Firefox still won't start, you need to delete any lock and .parentlock files left in its profile directory. There is a local command unlock-firefox for this.

mozillaZine has a more detailed article on this topic which may be useful if you have similar problems on your own computer.