I'm not sure how to submit bug reports, but this seems to be the place. Given it's nature, it's hard to reproduce, but I've got some pretty good info. Rather than repeat myself, see here:
http://forums.informaction.com/viewtopi ... 97&p=22137
Possible Bug: Update failure causes uninstall on FF 4.0b6
Possible Bug: Update failure causes uninstall on FF 4.0b6
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b6) Gecko/20100101 Firefox/4.0b6