In my case, selecting Firefox->Preferences->Privacy had 'Use custom settings for history' and the last item "clear history when Firefox closes" was set for some reason. I unchecked that, and now my previous session comes up (assuming I selected 'save' when I last quite firefox).

I have this issue if I hit close button to exit Firefox. When I hit Cmd+Q to exit, my session is restored successfully next time. It was happenning on Snow Leopard as well.

Yes, this is definitely a problem on Lion. What happens is a combination of both Lion and Firefox being dumb in opposite directions. Lion has this thing if you hit the "close" button on the window it doesn't close the program __unlike every other os, even other macs!!__ but it _looks_ like it closed. Since firefox doesn't count the window closing as a real close, it doesn't save anything. So if you hit the red "X", then right-click on the firefox icon and say "quit", and then open firefox, you lose everything. No session to restore from last time, nada.

Although you can try to always remember to right-click and say "quit", I really think firefox should fix this so the user experience is consistent across all OS's. That is, if you hit the red "X" on your window on your mac, it should save your session for when you open it again. Please firefox, pretty please.... :)

I have to switch between windows/linux/mac all day, so trying to remember different habits for each is a big PITB (don't get me started on "End" and "Home").

Every time I update flash on a mac (god I hate I flash, but necessary for some things), I "close" firefox, and I end up losing months of curated tabs that are "things I need to read eventually". It really, really sucks.