I always have automatic updates enabled, using latest Firefox version. I have enabled the options 'Open Previous Windows and tabs' or in earlier versions it was 'restore previous sessions' on my system windows 10 pro from past 2 years. I have lost about 15+ precious tabs in firefox (1 week of my research work). Even in history it is not showing those tabs. I was doing my research work / thesis, gone through about 500+ webpages / articles and selected the best, and aligned the tabs in sequence. Today i have opened the 'Private Window' doing some surfing, After few minutes I closed my 'Firefox window of research' as that was using 'too much' amount of ram. After 15 minutes i have closed the 'Private Window' also. When i opened firefox again all my tabs of Research Windows (1 week of my work) are gone, not even in history. I must be using Google Chrome for that because chrome does not have any issues with 'Restoring Sessions' with 'Incognito Window'.