The list of open tabs at last closing is stored in the profile file session.json. It may have become corrupted, in which case you could rename or delete it, with TB closed. It's automatically recreated when TB restarts. Also possible you have an app like CCleaner or Privazer that periodically resets files like session.json. In normal cases, a TB update doesn't affect the file.

Thank you for this, sfhowes--I am not a techhie person, so can you guide me a bit?

Q: Should I rename it? If so, what should I call it? Q: Or should I delete it? Which is best? Q: This has happened several time in the last few months, and it is heartbreaking to lose my tabs. Any way to avoid it? Q: CCleaner? how can I find it and disable it?

Here are the session.json files I have found (attached)--what should I do with them?

Rename session.json to session.old or just delete it, while TB is closed. Restart TB a few times and see if the opened tab set is retained.

CCleaner is a 3rd-party app that should appear under All apps/programs if it's installed. If so, disable any effect it has on TB, or uninstall it if it's not needed for other purposes.

I have not performed this maneuver yet, the tabs are seeming to be retained for now. About every two or three weeks I lose them all. Could it be related to a power outage? We occasionally have mini outages that last a second or two, and that may be when the tabs are lost. To that end, I have installed an uninterruptable power device to keep my system going in case of an outage.

Yes, power outages would have an effect.

Unlike Firefox, in Thunderbird the session information is ONLY saved at successful shutdown. If there has been no controlled shutdown then no session information is saved.

Yes, this is very helpful! Thankyou