I'm recently suffering a very annoying problem. Some times, after closing Firefox (cleanly, or due to a crash), I lose my previous session, and I have many tabs. I try the "obvious", which is copying one of the backups in sessionstore-backups, or the sessionstore.jsonlz4 from a working backup. None of this works. Even if I remove all sessionstore* files and directories and copy only the "good" sessionstore.jsonlz4, I get an empty (or 1-tab with home page) session. The only thing that has worked so far is replacing the whole profile directory with the most recent backup (and with this backup the previous methods seem to work).
I tried saving a session with the session manager extension, I can open it with the "working" profile, but when I try with the "broken" profile I just get a new empty window. What could it be that's causing the problem?
I'm still using version 56.0 (on Kubuntu), I know it's outdated and I should upgrade, but I need some quality time for that to find appropriate replacements for legacy extensions.
ETA: I don't know if this could be a red herring, but it looks like just leaving a session open for a while turns it into a broken one (meaning that after closing it, it won't restore the previous session). I disabled automatic addon updates, and it appears to at least make it last longer, while checking for updates (not applying any update, just checking) breaks the session again.
ETA2. That worked for a while. But not anymore. Now even selecting "Restart" causes an empty session. I may be able to find out the culprit, but this is becoming hopelessly unusable.