I can't advise you from personal experience - I haven't yet received the "invitation" to migrate from Comcast, and I'm happy to put it off as long as possible - but I can point you to the two articles, one from Comcast, the other from Yahoo, that supposedly describe the process. Have you read them carefully?

Upgrade your comcast.net email experience to Yahoo Mail:

https://www.xfinity.com/support/articles/yahoo-email-migration-overview

Information for Xfinity Comcast.net email accounts moving to Yahoo Mail:

https://help.yahoo.com/kb/new-yahoo-mail/SLN36803.html

Thanks for the reply. I've read both articles and still can't make it work.

What, exactly, happens when 'it doesn't work'? Please quote any and all error messages you see, completely and verbatim. Screen shots would be OK.

I hate to complicate matters, but blame Yahoo. Apparently they have introduced a new twist to authentication that affects some Thunderbird (and other email agents, presumably) users. I have multiple Yahoo accounts, but I have not been bitten by this yet, but I expect I will be eventually, probably if/when I am forced to migrate from Comcast.

Here is what Thunderbird has to say about it:

https://support.mozilla.org/en-US/kb/thunderbird-and-yahoo

If you have more than one Yahoo account, this part may be important:

If you have more than one account, such as one AOL account and one Yahoo account, to allow PKCE to work you must:

1. Click ≡ > Settings > General.
2. Scroll down to the bottom and click Config Editor...
3. Click "Accept the risk and continue" if that appears.
4. Type mailnews.oauth.usePrivateBrowser into the search box.
5. Click on the Toggle (double arrow) button to change the value to true.
6. Quit Thunderbird and then restart it.
7. Retry logging in to your accounts (Yahoo, ATT, AOL).