Restore all tabs from last session after restarting firefox

Always restore sessions on start

By default this feature is off, but can be turned on from the Options/Preferences menu. In the Main panel, under Startup, set "When Firefox starts" to "Show my windows and tabs from last time". This will have Firefox restore your tabs and form data to their state when Firefox closed (the related preference is browser.startup.page).

Note: Taken from http://kb.mozillazine.org/Session_Restore