What do you do if Firefox refuses to start? The error dialog says "Firefox is already running but not responding." I've run into this problem a few times recently. There is no Firefox process running, but it won't start.
Solution: Delete the .parentlock file in one of the subdirectories of the .mozilla directory.
Apparently this fix is described in the Mozilla 1.5 troubleshooting guide, but a search with the text of the error message didn't produce any useful results.