There are a few possible causes for this, full details are in the "Firefox is already running but is not responding" error - How to fix article.

Thanks TonyE, after changing my Profile, I am now able to start Firefox again. 8-) However, do you have any ideas how I can retrieve my Bookmarks? (Grrrrrh, I didn't back them up manually, are they auto backed up, do you know?) Cheers Muggins aka Chris 8-(

You can recover data from the old profile, see Transferring data to a new profile.

Thanks again TonyE for being patient with me when I should have RTFM! I have followed the procedure and got most of my data back. Thanks again, Cheers Chris 8-)

OK i tried all things,restart, uninstall, taskmanager, new download, Delete folder, Delete all fifefox,checking profile, and even after each one of them and reistalling I still get same message? Firefox also does not like facebook, always crashing durring it.

You do still have a Firefox Profile Folder ?

If all Firefox profile folders are removed but there is still a file profiles.ini that links to a non-existing profile then you get the same error message.

See also "Hang at exit":

• http://kb.mozillazine.org/Firefox_hangs
• Firefox hangs or is not responding - How to fix