I was in the middle of a very complex Internet session with more than 15 or 16 tabs open... and then Firefox warned me that my browser was dangerously out of date (and presenting a severe security risk). Since some of those open tabs were related to financial transactions, I accepted the update (which normally restores my entire session). This time, new Firefox offered to restore and sync even more stuff (bookmarks etc), which I did. However, once complete, Firefox opened to a new session, with nothing restored. Worse yet, my history had been wiped, restore previous session was greyed out, and there was no way for me to recover anything.

Boo, Firefox! This is a low blow, and poor user transition handling. I would expect this sort of brutal upgrade from cheap web sites, but not from Firefox.