Please do not use Session Sync for auto history saving. It's not something that it's guaranteed and it's even in the description of the addon on the first line with bold. It's just a feature that is most of good enough but not something that is perfect.

THIS ADDON IS NOT INTENDED FOR AUTO-SAVING OF SESSIONS!

As for saving sessions manually, those sessions are stored into bookmarks so it should be impossible to lose them unless the whole Firefox profile is broken in which case is not the addon fault.