But before you resort to force quit or the Terminal, here's the simple way out: manually close every tab and the main browser window. Quit will now work as expected.
Update: As noted in the comments, this isn't an issue with Firefox, but rather with the (amazingly useful!) Download Status Bar extension. The solution above works, or you can simply disable the Download Status Bar extension, and quit functions as it should. Since I find it so valuable, though, I'll live with manually closing the windows for now.

