This maybe is not the answer you expect - but did you tried to generate an app password? Till someone can test your case you can check the details from here https://support.mozilla.org/en-US/questions/1304518#answer-1349911 with yahoo link.

? I'm on a desktop workstation, not a cell phone. I do not have a cell phone. No "app" is involved.

By the way, my guess is that the browser-like window is Firefox 80.0, my default browser. The only other browser I have is Tor.

Are cookies allowed in TB Options(Edit/Preferences)/Privacy? No need for an app password, just enter the account password in the OAuth browser window. Also, set OAuth2 for both the incoming and outgoing servers.

Thank-you.

I fixed the cookies setting. Then I had to wade through 3 screens of sets of teeny images to proving that I'm not a robot. Then I was asked for my password. Now I'm in.

I saw only 1 OAuth2 setting, not separate ones for incoming and outgoing.

I would like cookies to be allowed only for these yahoo OAuth2 logins, blocked/forbidden/rejected for everything else. How do I do that?

Click the Exceptions button in Privacy/Web Content settings to fine tune the cookies.

The incoming authentication method is under Server Settings in Account Settings, and in Outgoing Server (SMTP) at the bottom of the left pane of Account Settings.

Found both authentication settings. They're now both set as you suggested.

What URL should I enter for the exception?

I closed and re-opened Thunderbird. I did not have to log in to the e-mail account. Is that the correct behavior?

When the authentication is completed, a token is stored in Saved Passwords, and applied for each login automatically.

I think *.yahoo.com or login.yahoo.com will work for the cookies exception.

Hmmm...

Things must be slow for you tonight. I appreciate your quick and helpful replies.

All seems to be working at this time. I'll give this another day. If after the overnight shutdown and morning boot, no further problems occur, I marked the problem solved, and start doing the other yahoo accounts.

In the yahoo e-mail accounts that are not yet "converted", almost every time I go in to a different folder, send a message, or even open a message, I have to re-enter the account's password. This get tedious. I'm hoping this problem will go away once I've "converted" all the other accounts. So let's leave this aside for now.

The "converted" yahoo account is working fine. So I consider this thread's problem solved. Thank-you for your help.

Two enhancements suggestions for Thunderbird: 1. When I launch Thunderbird, I'm automatically signed in to all "converted" yahoo accounts. I would prefer to choose. A simple yes or no on each account should be sufficient. If later I want access to an account that I said "no" to, I could be logged in by simply clicking on the account name in the left part of the Thunderbird display. 2. (for all e-mail accounts, not just yahoo accounts) I know of no way of logging out of a specific account. I would have to close Thunderbird, but that logs me out of all the accounts. Please provide a way of logging out of a specific account. If I should submit an enhancement bug for these, please let me know.

I don't know of a way to log out of accounts individually, but a POP account is easier to set to not log in, compared to IMAP accounts. By disabling checking for mail at startup and periodically, mail can then be checked manually on a per account basis.

As of today (Oct. 22), my yahoo accounts seem to have been behaving properly for 3 days now. The conversion to "oauth2" worked. Thank-you, everyone, for your help.

I needed to add "https://accounts.google.com" to the "Exceptions..." in the "Preference" > "Privacy & Security" to get OAuth2 working again after #googledown. This solution allows you to leave the "Accept cookies from sites" unchecked which might allow any site to store cookies.

• https://stackoverflow.com/a/63255601/3553808 : Access WebMail Type Of Email-WebSites From TB-WebBrowser-Tab , or , Configure TB to allow very specific OAuth2 based website-addresses in TB's Cookie-Exception list, etc, etc. (please upvote my Q+A both, inside the linked page, if its useful/helpful).