Session Restore
Allows Firefox to start off where you were when you closed it, keeping all windows and tabs intact.

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Tweak Type:	General |Tabs
Fx 2:	YES
Fx 3:	YES
Fx 3.5:	YES

Starting with Firefox 2.0, Session Restore allows you to restore your Firefox session to exactly the way it was prior to the browser crashing. It can also be changed to work like Session Saver and Tab Mix Plus, in that when you reopen the browser it will be exactly the way it was when you closed it last. This use to be an about:config tweak, but has since been placed in the options menu.

To have Firefox to restart where you were when you last closed it:

From Firefox Tools Menu select Options…

From Main in the Startup section, When Firefox starts: select from the drop-down menu Show my windows and tabs from last time

Click OK

Source