Are you using OAuth2 authentication, and do you get the Yahoo sign in window?

Yes, it is set up to use OAuth2, and yes, I get the Yahoo sign in window, which I go through -- I type in my Yahoo password and give permission for TB to access Yahoo. And Yahoo acknowledges the attempt to sign in with an email that says that Thunderbird attempted to sign in. As I said, everything is exactly the same as on the laptop -- which works. The configuration, username and password are exactly the same, so I don't know what to do. Thank you for showing interest in my issue.

So, at which point do you get the "Unable to log in at server. Probably wrong configuration, username or password." message?

Thunderbird home: FILE>>NEW>>EXISTING EMAIL ACCOUNT

Next page: enter my name, my Yahoo email address, and my Yahoo password. Then "CONTINUE"

Next page: click on POP3. I have always used POP3 and will continue to do so. TB looks and then produces in green box, "CONFIGURATION FOUND IN MOZILLA ISP DATABASE." Then, "DONE"

Green box turns into white box with "CHECKING PASSWORD." Yahoo window opens, with my yahoo email address. I hit "NEXT"

Yahoo window next page: asks for my Yahoo password. I enter it and check to make sure it is correct. Then "NEXT" It is at this time the error message appears.

Yahoo says Hi to me, and I "AGREE" to the Yahoo "Open ID and OAuth terms," and I do -- but by this time the error message has appeared.

Just to create greater confusion, I went through this installation on a third computer at a different location, and it worked perfectly there. So there is something goofy about my desktop, where I have accessed Yahoo mail for years through the Yahoo website. I was trying to get Yahoo mail onto Thunderbird because I have more than one email address and I want a place where all emails appear at once.

Thanks.

Something is definitely goofy about your desktop. Which antivirus/firewall solution are you using? Quit Thunderbird then delete the %AppData%\Thunderbird folder. Restart Windows in safe mode with networking and launch Thunderbird. See if you can setup the account without facing that problem. Since you're using OAuth2, skip enterering the password in step 2, that is, just enter your name, email address then click Continue.

Your solution worked! I uninstalled Thunderbird with a nasty uninstaller that cleans up the registry (IObit) and then re-installed it. I tried to install yahoo on TB and omitted typing the password as you said. It worked and now I have yahoo emails on desktop Thunderbird.

I would never have thought to install yahoo without keying in the password. Either that, or the hard uninstall, made the difference. Thank you very much!

It's omitting the password, because the password you provide in that field/textbox is used to verify that you can sign in to the mail server via the Normal Password authentication method (basic, insecure legacy method), while OAuth2 authentication is a completely separate process. Yahoo no longer supports basic authentication using your Yahoo main password, so that verification process failed with that error message, while OAuth2 worked just fine.