Is it this one https://support.mozilla.org/en-US/questions/1581751#answer-1818548 ''<p>Phil [[#answer-1818548|said]]</p>'' <blockquote>I believe I have some insight on this issue. I have been working for weeks to setup an account for a defunct comcast.net email address that was taken over by Yahoo. I found the following in another report: "Try to delete the password Thunderbird may have remembered for the troubled account. Then Thunderbird will prompt you for the password when it's needed. At the top right of the Thunderbird window, click the menu button ≡ > Settings > Privacy & Security > Passwords > Saved Passwords" When I followed the instructions the "Saved Passwords" popup showed the provider account, the email name, and a date, which I surmise is the creation date. Could it be that Thunderbird stores the passwords by provider? </blockquote> No Thunderbird stores passwords per account with separate passwords for the incoming and outgoing servers on an account. Please refer to https://support.mozilla.org/en-US/kb/password-manager-remember-delete-change-tb <blockquote> That might explain why I get "Authentication error" when I try to save an account such as "lastnamefirstinitial@comcast.net" as a yahoo account; it's using the yahoo.com password which is not correct for the comcast.net account on Yahoo. If so, I think Thunderbird needs to store passwords according to the email address instead of the provider account. I hope that helps. </blockquote> More likely if you are using oauth authentication is that Yahoo in their insanity share the same cookies for all accounts, so you have to force Thunderbird to use a Firefox style private window so it can not do that. Refer https://support.mozilla.org/en-US/kb/thunderbird-and-yahoo where it states # Click ≡ > Settings > General. # Scroll down to the bottom and click Config Editor…. # Click Accept the risk and continue if that appears. # Type mailnews.oauth.usePrivateBrowser into the search box. # Click on the Toggle Config Editor Toggle button to change the value to true. # Quit Thunderbird Click ≡ > Quit and then restart it. # Retry logging in to your accounts (Yahoo, ATT, AOL).