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

Hi. This problem just happened again. I am supposed to be on the esr release, but Thunderbird just automatically upgraded me to the version 133.0. It puts up a red flag, saying I probably shouldn't be running this, and should download the last stable version. It points me to version 128.5.0, which is likely what I was running before. But now I have the same problem. If I try to use 128.5.0, it will complain that I am downgrading, and my profile won't work. Should I take the suggestion in the link above, to go ahead and try to downgrade using the command line with the allow downgrade option?

And why does this keep happening to me?

Thanks for your help

I think I have a clue about this. My university does control some updates on my machine. I asked them specifically about Thunderbird, and they said no, they don't push Thunderbird updates, but they do push Firefox updates. I checked the dates, and the dates that I get the Thunderbird "release" (not esr) updates correspond with the Firefox release dates. Could these updates be linked, so that my Thunderbird also gets updated, even though the university doesn't realize it? Can this be controlled in some way? As always, thanks for your help!

> they said no, they don't push Thunderbird updates, but they do push Firefox updates. I checked the dates, and the dates that I get the Thunderbird "release" (not esr) updates correspond with the Firefox release dates. Could these updates be linked, so that my Thunderbird also gets updated, even though the university doesn't realize it? Can this be controlled in some way?

IMO too much of coincidence that they push Firefox updates. So I think they should recheck how they might be affecting Thunderbird.  :)