I think that the friend list gets deleted whenever Firefox is upgraded. The thing is I always forget to do a before/after check, but I noticed a day later it got deleted when I updated to 3.6.4 recently, and this morning I updated to 3.6.6 and it disappeared again. Can someone who hasn't done the upgrade yet try it and confirm?
Scratch that, the friend list actually gets deleted whenever you delete a friend with the X button from the friend list. If you delete him from his profile page, friend list survives.