Please read this article to see if it helps:

http://support.mozilla.com/pt-BR/kb/Firefox+hangs#Hang_at_exit

I'm quite familiar with the issue you've proposed via this KB article. My issue, though, is quite different. I don't get the annoying "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.." alert box.

Further, with the known issue, the firefox process does eventually die. With this new issue, the firefox process never dies and appears orphaned.

Keep in mind this only started occurring after I upgraded to version 3.6.9 two days ago. There is no doubt in my mind something is broken in the newest build.

As a developer, I like adhering an important and well-known principle in the software development world when I build and maintain stuff: if it ain't broke, don't fix it. Sometimes I wonder/question if Mozilla developers hold the same conviction, given numerous new issues that make it into every new build of Firefox.

This also brings up another subject: does anyone do QA anymore? I like to test my stuff before letting others use it. It's extremely important to make sure your stuff is solid, before letting others use it. Cross your T's and dot your I's, for heaven's sake!