Did you file a bug on this? If not, could you do that please? Since people aren't seeing this in general, your help would presumably be needed to narrow this down....
Actually, I've been thinking about it and the problems happened around the time I upgraded to Windows 7, at which point I installed Firefox to my (relatively old) SSD, which was problematic in the past for Firefox. I'm going to reinstall it to my HDD as I had it previously and see if that fixes the problem.
