These two Comcast articles may prove helpful:

Comcast requires you allow access to third-party apps such as TB: https://www.xfinity.com/support/articles/third-party-email-access

If you've done this before with one PC, it might be necessary to repeat it for other devices with the same account. Check also that the server settings match the current recommendations: https://www.xfinity.com/support/articles/email-client-programs-with-xfinity-email

The first site walked me through enabling third-party access. It was already enabled (did I mention that it had been working for years?) The second walked me through setting up a new account. (did I mention that it had been working for years?) Does this mean I should try deleting the accounts in TB and setting them up again? Is there some other magic I'm missing? Any idea why it suddenly broke? (did I mention that it had been working for years?)

You are right. But there are many who post having changed passwords and found they were supposed to use app passwords. So, I sometimes over respond.

No problem. At least you listened to the actual problem, as opposed to the automated systems with their extremely limited set of "solutions". I guess I could try removing and reinstalling TB next. But that's a real pain as I lose all my event reminders.

So I removed the existing account and tried to create a new one. Won't take. Keeps saying it can't validate username of password. I'm extremely frustrated. Can't bank my head against a wall for more than a half hour at a time, so I'll try again later after dinner.

I have the same problem!! I think it started on the 19th when Thunderbird did an update. Apparently the problem with a free app like Tbird is they don't even try to have any support staff. Like you I've used it for years with no problems. I hate to abandon it because I have so many contacts with this address. I may try to get Comcast to open an account with that name but they are, and always have been, useless at support which is why I've been using TBird. Ugh.

Kurt, I share your frustration; I see multiple Comcast problems posted here regularly. I did see this URL but probably not your issue: https://www.xfinity.com/support/articles/two-step-verification-xfinity-app-setup