Firefox lost restore previous session setting

[Gary Dale]

I'm running Stretch on an AMD64 system. Lately I haven't been able to 
locate the restore previous session button and Firefox doesn't seem to 
want to do it automatically either.

Has this option been removed? I find it terribly annoying to have to 
reopen all the tabs I normally have every time I restart Firefox, which 
I have to more frequently than I like because it gets excruciatingly 
slow if left running for a few days.