You issue is almost as old as Thunderbird. the Common cause is anti virus software blocking or partially blocking the new version of Thunderbird. Norton in particular appear to do this. But it probably not alone. A check of the firewall and other settings on the anti virus is probably required. Or wait a few days. these programs phone home daily and an update usually "self heals" the issue in a day of two from update. Or so our users tell us on the forum.

It started working again. (This is several reboots later, including a Microsoft Update.) But now it wanted to use Edge as my browser even though I had set it to Firefox in Settings -> System -> Default Apps. However, going Control Panel -> Default Programs fixed it. Still seems like a mystery to me why it broke and then fixed itself without changing the Thunderbird version.

Because the problem was not Thunderbird. It was something else, probably your anti virus. They update daily or Windows even, as you point out it updated in the middle of your problem

I kind of doubt it could be due to antivirus. I just use Windows Defender in WIndows 10, and it is pretty benign. Other programs were still able to send URLs to Firefox. Quite frankly, I am at a loss to understand how general passing of a URL to a browser could be an activity with which an antivirus program would have any reason to interfere. I could imagine a sort of protection which is aimed at specific URLs, but that is not what was going on here, since, in my case, any URL was failing. Furthermore, that sort of protection is more easily managed in the browser.

In any case, I do appreciate your response. Thanks.