Firefox is already running but not responding

It's an old issue (several years, at least), but it still happens. Ocasionally, the Mozilla Firefox browser will fail to start, citing the error: "Firefox is already running but not responding". 

Other instances of the browser may of may not be running.

What you do, is pop up a terminal window, and type the following:

$ killall firefox$ rm -fr $HOME/.mozilla/firefox/*default/.parentlock

After the .parentlock lock file is gone, firefox should start as normal.

On shared machines with many novice users, I'd put the following in $HOME/.bashrc:

alias firefox='rm -fr $HOME/.mozilla/firefox/*default/.parentlock; firefox'

That way, everytime you run firefox, the lock gets deleted first. In my experience, there aren't any noticable side effects. But do drop me a comment if you find any.