This is a Firefox extension that auto-saves all tabs when you exit and reloads them when you restart Firefox. Very nice. You probably want to read the user comments at the site linked below, because it sounds like it may have a few weird behaviors (not to mention the confidence-inspiring "version 0.2" label ...).

Link: Firefox Session Saver

The "Developer notes" on the above site recommends getting a newer version from the author's homepage, but it seemed out of date. Hm. The linked version works fine for me.