Bugzilla

With help from the information in this ticket (thanks!), I managed to solve the problem.

After seeing the calendar-related errors in the error console (similar to, though not exactly the same as, those mentioned in comment 19, I investigated. I'm not using the calendar, but until a few years ago I did, and I had symlinked `calendar-data` to somewhere else, and it was now a dead link. I removed that link and started Thunderbird (still 91). It created a new `calendar-data` directory alright and stopped showing related errors in the error console. I then proceeded to upgrade Thunderbird and everything seems to have worked fine.

Just to be clear, this wasn't any trivial disruption. A few months ago, when it automatically updated Thunderbird and everything disappeared, I had to manually reinstall 91, find out how to stop it from automatically updating, and bring back my destroyed profile from backup (although I have everything on IMAP, recreating these accounts would mean I'd lose my message filters and who knows what other stuff [maybe years of spam training?]). I found this ticket and kept an eye on it. Today I felt like giving it a careful fresh try. For less technical users this can be a great pain. I admit that it was my being technical that triggered this bug in the first place. Still, it's a critical data loss bug.