Electricity cut off suddenly, firefox crashed, closed firefox and opened it again
And found the restore session "about:sessionrestore" !!!!
hate it , whatever the reason was
here are some easy steps to follow:
(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.
(2) In the search box in that tab, type or paste sessions and pause while the list is filtered
(3) Double-click browser.sessionstore.resume_from_crash to toggle its value from true to false.