Very costly. Many things can go wrong such that the so-called "previous" session is not the hundreds of tabs I actually had open but just one tab, consisting of the Firefox start page. Apparently Firefox does not save older sessions (such as 3 or 4 such sessions). Sometimes the "restore" button only restores the one tab, not the actual previous session. "Restore previous session" is greyed out under the "History" button. Therefore I am sunk. This failure to access older sessions has happened to me dozens of times and has cost me thousands of dollars. I would like to see 100 million people advise Mozilla that Firefox browser should automatically save older sessions (such as 3 or 4 such sessions), and make them available to restore. In the meantime I am maintaining an MSWord file in which I manually record all of my open tabs, so that this won't be so costly the next time.