I too have something like this problem. When WebShield is running, Firefox 3.0.1 (running on XP Home) can take up to a minute to close. With WebShield disabled, 10 seconds. Very irritating, especially as I had no problems with Firefox 2!!!
On the Mozilla Firefox Bugs forum I was initially advised to install TCPView to see what was going on. From observing what appears to be going on in TCPView, it seems to me (I consider myself a non-techie here) that Firefox, WebShield, and a related System Process are all scrapping over Port 12080. As soon as all the connections to that port close - and as I said, it can take about a minute - Firefox closes very rapidly.
Any suggestions anyone, please?