You should follow the instructions you received from Comcast/Yahoo. https://www.xfinity.com/support/articles/yahoo-email-migration-overview https://help.yahoo.com/kb/new-yahoo-mail/SLN36803.html

Please quote any error messages you see in full, verbatim. Since the switch from Comcast to Yahoo is voluntary, I won't be doing it until forced to, if ever. But it should be possible. You may have to enter the server information manually because Thunderbird may suggest the wrong settings based on the Comcast address you use for a logon ID. You may have already seen these articles about the move, but it's worth scanning through them again, especially the Yahoo one, which has the new email settings. https://www.xfinity.com/support/articles/yahoo-email-migration-overview https://help.yahoo.com/kb/new-yahoo-mail/SLN36803.html If you will have multiple Yahoo accounts in Thunderbird, you might need to go through this procedure (though I have not had to): https://support.mozilla.org/en-US/kb/thunderbird-and-yahoo