Sometime , firefox did crash on me for don’t know what reason.
I think is because of Flash.. It was making some weird sound when tried to play a youtube video.

But even when pressed the close , exit button. (Quit_)
Upon trying to re-launch the firefox ..

something like this appear.

“there is another firefox process already running “

Hence, need to kill all Firefox process one by one.
So come the following …

$ ps xa| grep firefox| grep -v grep | awk '{print $1}' | xargs kill -9

So, after that you can re-launch the firefox again.
As it would no longer complain..