I have an update on this. For some reason, although I am supposed to be on the esr release schedule, a beta version, 131.0.1 was downloaded to my computer on Oct 22nd. Then last night, the 128.4.0esr version was downloaded. So indeed, there had been a newer version (or at least higher number) release on my computer, so the 128.4.0esr version didn't like the profile. I used time machine to get my latest profile with a 128 release, and that basically fixed it. But there does seem to have been an error on Mozilla's part, because I never signed up for the beta program.

Can you post a screen shot of Help > Troubleshooting Information > Show update history

I tried this. The update history shows the last update as 128.3.2esr, installed on Oct 18, 2024. This is because in fixing the problem, I replaced the whole Thunderbird folder in the Library with an older version, from Oct 19. The information from above on version numbers was from using time machine to check what version was there on different dates.

Maybe I shouldn't have replaced the whole folder, and should have just replaced the Profiles folder. Any advice on how best to set up that folder would be appreciated. Things are working now, but I worry they might be in a funny state. I did make backups of that folder from right after the problem, right before the problem, so I could replace some of those files with newer versions if that would be better.

Thanks for your help.

A "drastic" step is to use an older version with a profile that has been used on a newer version - for example going back to 131 to version 128. You can't always expect that to work without problems. For example users should not go back to 115 from 128 because the folders have changed.

However, at the present time there has not been enough change between 128 and 131, so you could have gone back using https://support.mozilla.org/en-US/kb/dedicated-profile-thunderbird-installation#w_what-happens-to-my-profile-if-i-downgrade-to-a-previous-version-of-thunderbird