I've been looking for a FireFox extension that will remember what tab/web pages I had opened after closing and re-opening browser instances. I found one yesterday after searching, again, (I've looked for something like this for sometime now) through the myriad of extensions - in the Navigation, Tabbed Browsing sections. I give you...
SessionSaver. This extension seems to do exactly what I've been looking for. I have not experienced any issues with installing subsequent extensions like a couple of the comments I read on the extension download page. Give it a shot.