If "Restore Previous Session" is gray on the History menu, then the about:sessionrestore page will be blank the way you show it in your screenshot.

Did Firefox already restore the previous session?

I have the same problem. I am running Firefox 50.1.0 on Ubuntu 16.04. I had a very large Restore Session (in fact a nested string of them) which has for some reason stopped appearing this time around. I don't know what preceded this. What, if anything, can I do?

Hi penthimos, I replied in another thread you joined, over here: https://support.mozilla.org/questions/1151274#answer-950725