Problem: If you close browser and open + ctl shift T, we get back last session.
But if you close( session with 5 tabs), open again and did not restore , but open a new site and close. Open again, you will not be able to get back those 5 tabs.
This is not good. the history should exist till it is used ( ctl shift T). may be after one use , firefox can assume that user does not need those tabs.