Additionally, attempting to install version 128.3.0esr, the "latest stable," now results in a warning that my profile is no longer compatible. How am I supposed to untangle this mess?

click to settings>general and scroll down to the updates area and click 'show update history.' My guess is you were using version 125.0 or 130.0, neither of which are accessible from thunderbird.net for download. There are some software distributors, such as Majorgeeks, that offer unsupported versions. My guess is that you got the prior version from such a place. If the default updates did this, there would be tens of thousands of users experiencing it. My suggestion is to install the latest release and, if you receive any compatibility messages, to look for and delete a file called compatibilities.ini

Update History claims that none have been installed. The screenshot is attached, because that is hardly believable. I've been using Thunderbird on an almost daily basis for well over a decade. I still remember Netscape!

I can say with certainty that in May, 2024, I had version 115.10.2 on the ESR channel. I know because at that time there was an update that caused the ESR channel release to display the warning message for the daily channel. Wayne Mery diagnosed this as a glitch.

I very much doubt that I have ever installed from a source other than Mozilla. My computer is managed under JAMF, so it is possible that my IT department pushed something strange. I'm checking on that.

What exactly will deleting compatibilities.ini do?

the compatibilities.ini causes you to be prompted that your upgraded version won't work. it's intended, I think, to sensitizes people from going backward on releases. Deleting it removes the warning. I have seen other IT groups doing this, thinking that they're keeping users with most current release, but it doesn't work. They should only use thunderbird.net. We have seen several posts similar to yours on 125.0, 130.0, and 131.0. I wrote to Majorgeeks on this and they now include a comment that the release may not be supported.

I believe you're on the Release channel and are not on the ESR channel. In Settings->Config Editor, what does 'app.update.channel' show?

@david: Does this mean that (deleting compatibilities.ini) is equivalent to running Thunderbird with -allow-downgrade? (I didn't know about either of these options when I first posted.)

@Corey: Aha, thanks! I had deduced that I must be on the Release channel (based on the versions served at https://www.thunderbird.net/en-US/thunderbird/all/). But I was searching for where I could determine my channel directly. I'm surprised this is not plainly indicated in the "About Thunderbird" panel.

Indeed, I have app.update.channel=release. Not only that, but that parameter is locked from editing. Is that normal? Does this explain the empty Update History, resulting from a channel change?

My mystery now is how I got onto the Release channel. david's theory about IT departments seems plausible for my case. I gather that the Release channel did not exist until ~1 month ago. Is it correct that the only way anyone could be on the Release channel is by installing v.131.0 during the past month?