See http://kb.mozillazine.org/Session_Restore

This was not helpful at all. I don't have any of the files listed in these paragraphs and when I go to "about:config" none of the preferences listed in the solution are present on that page. I don't think that solution was written for version 3.6.12. Please try again. Thanks, Steve Rogers

actually it does work, just firefox named it differently. Tools, Option, Start up, on "when firefox starts up" choose "show my windows and tabs from last time."