How to restart Firefox after it crashed

If firefox crash and once you click again on his icon, refuse to start, claiming that there's another firefox instance running, but YOU DO KNOW that is not true, use this solution:

Open a terminal window
cd $HOME/.mozilla/firefox/<random string>.default/
rm lock
rm .parentlock

after deleting those files, firefox will start again.

<note important> The same rules are valid for all the software provided by Mozilla Foundation as Thunderbird, Mozilla, Seamonkey. Just find the correct subdir, before removing the files. </note>