This error is being issued by your providers email server. Thunderbird is just delivering the message. Everything that I see about this error code points to Yahoos 2 step verification system. You need to work this out with Yahoo. Comparing your ability to log into the web mail page and using email servers is meaningless. Well almost. It does verify that you know your username and single step password. Other than that it uses a complete different set of servers and log in processes.

Bill9145 said

I have followed the directions to set up my account but continue to get the message (#MBR1240) Please verify your account by going to https://login.yahoo.com) ... my account and password work just fine on the yahoo login page but thunderbird keeps giving me the same error message ... some times the # given will be BR1212 ... Help!!

I understand the problem is with yahoo but how/who do i contact to get is working properly? Contact Information for help?

About Second sign in verification: https://help.yahoo.com/kb/yahoo-account/SLN5013.html?impressions=true

About Imap setup: https://help.yahoo.com/kb/yahoo-mail-imap%C2%A0settings-sln4075.html

How to turn off second verification: https://help.yahoo.com/kb/SLN6050.html?impressions=true