Firefox normally only stores one previous session... were you possibly using the Session Manager extension before? Regarding your immediately previous session, if you have already used History > Restore Previous Session, you could check the History menu for previously closed windows (normally, Firefox remembers up to 3 previously closed windows, and up to 10 closed tabs per window). ---- During your last update, is it possible that you ran the Refresh function? This could occur if Firefox display a notice that it was performing poorly and you could speed it up by using Refresh. During that process, an Old Firefox Data folder is added to the desktop. Do you see anything like that?