Nope, it was blank, because the Plist files have nothing in it. And I cant rescue it either. In Firefox they make lastsession.json and lastoeesion.bak where .bak was a backup and could be restored from within UI setting. I think they also had the .bak zipped somewhere in case all things fails, you can manually restore some part of it.
Although the frequency of that zip creation was something like daily so you still lose some part of tabs. There were additional guarantee they put in place so last session will never be overwritten again or blank out. But that was a very long time ago when I was still a heavy Firefox users and follows its development very closely. If I remember correctly those features development and debugging happened on Mozillazine.
