Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.4; en-US; rv:1.9b3pre) Gecko/2008012310 Firefox/3.0b3pre
Steps to reproduce:
-> New Profile
-> Install Simple Mail Extension / https://addons.mozilla.org/en-US/firefox/addon/5593
-> Setup this extension with a Mail-account. For testing and reproducing you can let the password field empty.
-> Check mail, you will see a throbber down on the right side
-> Quit Firefox
Created attachment 298833 [details]
0 TOTAL 22 2268 789194 7 ( 1754.24 +/- 1583.48) 2292851 10 ( 3175.30 +/- 4601.38)
Minusing since the leak is only temporary. It won't build up over time.
This looks very similar to bug 412207