If your computer crashes while firefox is running, it's possible that you will get an error message like this when you next start up firefox after rebooting:
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 remove the lock files inside your profile. On Linux (with a default setup where you have a single firefox profile) this looks something like:
rm -f $HOME/.mozilla/firefox/*/.parentlock $HOME/.mozilla/firefox/*/lock