Actually, you're not the first person to mention this in the past few days, but I haven't come across enough instances to find any commonalities yet that could be causing it.
What seems to be happening is that the database crashes, and FF starts from the "default" profile. But on the next start, it rebuilds the database and stores the restored personal data in the "default" profile, leaving the previous user profile reverted to the FF defaults. (At least, that's what seems to be going on based on a very few incidents. I could be wrong.)
As to why, I have no idea. I don't know if it's a FF version bug, but it seems unlikely because because 2.0.0.6 has been out since late July. It could be an extension problem. Do you have any extensions installed?
Rich