Help/More Troubleshooting, Profile Folder, Open Directory, close TB, delete or rename session.json, the file that stores the last opened folder and set of open tabs at last closing. Restart TB a few times and see if it works properly. If not, close TB, delete session.json and xulstore.json, the file that stores the toolbar and window layout.

Thank you for trying to help! It didn't work. But in order to re-index the search database I had to restart TB, so I guess the file with the tabs is gone by now?

Rebuilding the search database isn't related to the session file, which is recreated every time TB is closed. If session.json is deleted, you will have to open the tabs again, but they should then be remembered when TB is closed and reopened.

OK. If there are no older session files, then the tabs must be gone. Generally there is no problem. The tabs normally reappear when TB restarts. The problem was that TB crashed for some reason.

Post a crash ID from Help/More Troubleshooting, Crash Reports. An ID is a string starting with bp-...

The newest crash report is from February...?? bp-d2da6c5d-4220-476d-80d7-679530210213

Crash reports older than 6 months aren't retained. See if running TB in safe mode eliminates the crashes (Help/Restart with add-ons disabled).

This recent crash is probably the first since February. I don't expect it to crash again, so I don't think there is any need to run in safe mode. My concern still is if there is any way of getting my tabs back, now that I have restarted TB a few times trying to solve things?

As mentioned above, you will have to open the messages again to keep them open when TB restarts. Consider other means of marking messages, such as tags or stars.

I didn't imagine that TB could forget all my tabs... It would be very nice if it remembered more than one session.