The problem
When starting Firefox (version 1.5), instead of the browser window, a window appears with the following error message:
"Firefox is already running but not responding. To open a new window, you must first exit the existing Firefox process or restart your computer."
But you are sure that Firefox is not running. Even a restart does not help.
The cause
Firefox was terminated improperly during the last session. This left a lock file in your home directory. Firefox then thinks that it is already running and does not want to restart.
The solution
Simply delete the lock file. The easiest way to do this is to use the following command, which removes the lock for all profiles at once:
user@linux> rm ~/.mozilla/firefox/*/.parentlock