Yahoo requires an App password, which is a password different from one allowing web access. More info on that at the yahoo website.

I'm experiencing similar problems since a few hours ago, with Microsoft 365 (worked perfectly about 6 hours ago). I've also used Thunderbird for these mailboxes for maybe a decade but now it won't log in, the problem persists on two laptops each running Thunderbird 91.11.0 (64 bit). I can access the mailboxes by webmail and using the Apple Mail client.

Earlier I posted about it in another thread (https://support.mozilla.org/en-US/questions/1382051), there someone suggested going to the Account settings/Server settings changing the Authentication method to Oauth2 but it didn't help.

I suggest you pursue with your email provider, as different requirements may exist. Changing to Oauth is usually not the only requirement. You may find that you need to remove existing password before being able to secure a new password. I'm not familiar with MS 365, but Gmail, for example, has several requirements in addition to setting the Oauth2.

Thank you david

Mozilla Support sends its email notifications to an email I no long have access to, so I have to check in from time to time. I can't figure out how to change my email address here.

The problem persists. Fortunately I can access one of the emails on Yahoo and the other on Outlook. I have two more emails for which I've used Thunderbird for years. They were both created by our local telephone company, which I no longer use as an ISP. One still works but Thunderbird has forgotten the password of the other. I suspect it may have been hacked. I replaced this email at dozens of website log ins with an old Yahoo email I had forgotten about.

I get this popup message: "Enter your password for *****@yahoo.com on imap.mail.yahoo.com:" After I enter the password, the error popup message repeats.

Another email was hosted by GoDaddy for years. GoDaddy recently switched it to Microsoft. After I enter its password in the password error popup, I get this message: "Login to server outlook.office365.com with username *****@b*****d.net failed."

I've used Thunderbird as my primary email client for years. If I can't fix this, I have little use for Thunderbird anymore. I appreciate any suggestions

First, to update your email address for this forum, click your profile and then the 'edit settings' link, which will then direct you to a Firefox page to update the email id.

On your problem, both Yahoo and Outlook require specific passwords for access from an email client, distinct from the password used online. This is not a Thunderbird problem; the problem is that Thunderbird is using the settings you provided and they aren't being accepted by the email host.