[bigbugbag]

Last time I tried the groups that were opened when quitting firefox would reopen at next launch but you could not "reopen" a closed group. Having more than one window was asking for trouble as in risking losing all your groups and tabs because there another firefox window opened in the background or the downthemall manager window when you closed the main firefox windows.

Workaround is to always quit firefox using the ctrl+q shortcut, though at times groups will reopen with the correct number of tabs but they're all empty.

Best bet is to manually save your session at times.