It is repeatable whilst ever you use the bug ridden mail server that is provided by Yahoo. See https://support.mozilla.org/en-US/questions/1209553 for the weird solution.

@matt -- I found and tried that solution without any success. If I use 'username', it works until it doesn't. Then I set it back to 'username@verizon.net' where it works until it doesn't. I can also go into the settings, remove the 't' from 'username@verizon.net', hit enter, put the 't' back, hit enter and it pulls emails.

While I go to some lengths to avoid Yahoo so I have no personal experience any longer (Life is to short to deal with Yahoo is where I am at). Do you have your authentication method set to oAuth. This is the preferred method for AOL and Yahoo.

Yeah, it's set to Oauth2 per AOL's instructions. TB was working until the windows updates: 2020-10 Cum Update for .NET (KB4578968) and Feature Update on Win10.

Uninstall the update(s).

@Stans -- tried that. Same issue. And windows said the prior update was more than 10-days ago and wouldn't let me go that far back.

Are there TB logs I can pull which will help?

I doubt Tbird logs would help much here, since this is an old Yahoo Business problem dating back to 2018 or even earlier, so this is not a Windows Update or Tbird update issue. If I were you, I would just remove the account (including message data) and add it afresh, strictly using the settings given here https://help.aol.com/articles/how-do-i-set-up-other-email-applications-to-send-and-receive-my-verizon-net-mail

@Stans, that did it. I needed to make sure it was setup as IMAP to get the sub-folders. Both of you, thanks for the help!