check you have cookies enabled on Thunderbird. Some software disables them "for you protection" the the required oAuth connection authorization can not occur.

please carefully listen to what Matt has indicated & pointed out.

then please check for my answer in below link/page, here (if its helpful, pls upvote). then check other answers.

if only TB was updated/upgrade, no other software, then you can skip below AV/FW/SS related 2 paragraphs.

some AV (anti-virus), FW/SS (firewall/security-suit), etc software can intercept email connections for extra protection, tweak those settings in your AV/FW/SS software and try to find a balanced solution where email check is possible via TB (Thunderbird), and AV can also check/scan for malware in emails.

( Enable only TB's internal option to allow scan for malware & removal. ) ( AV/FW/SS,etc uses various types of checking/scanning , i mention 2-types in brief only : (1) it intercepts emails by sitting in middle of internet/network connection, in between your TB & mail-server ... you should disable this kind of protection , when : you have private/sensitive materials & when privacy is more important/PRIORITY for you. (2) AV/FW/SS,etc can create protection by scanning/checking individual email(s) when+after TB stores/gets them as single file inside your own storage-drive ... you should keep this type of protection enabled . if you have private/sensitive files & privacy is PRIORITY for you, then do not allow AV/FW/SS to send hash-code or file(s) outside of your computer into remote computer for check/compare , that type of option assists to steal your privacy & assists to build anti-privacy databases , So select/unselect options so that AV/FW/SS,etc is scanning/checking emails inside your own computer (not in a remote computer), etc. )

and continue to check/search in this (support.mozilla.org=SUMO) website for earlier/OLDER posts related to Yahoo, & answers/solutions inside them, and also check back this page, which you began.

I have no 3rd party Av. Just using the standard windows firewall. The cookies are turned on. This problem is effecting only 1 out of my 3 Yahoo accounts. A year ago I could find a yahoo configuration that said allow clients with lessor security. Logging in to the account with the problem via firefox that parameter can no longer be found

Much internet research shows yahoo no longer has the allow less secure login for new accounts. They do not even allow login n password with imap. Oauth2 is required. They art trying to force everyone to use a yahoo client only. To even get a security code you need to use one of their 4 apps on your mobile. I am now changing al my email accounts

Much internet research shows yahoo no longer has the allow less secure login for new accounts. They do not even allow login n password with imap. Oauth2 is required. They art trying to force everyone to use a yahoo client only. To even get a security code you need to use one of their 4 apps on your mobile. I am now changing al my email accounts

So go to your account setting on the Thunderbird menu, select your yahoo account and actually change the authentication method to oAuth. Less secure apps applies and only ever applied to authentication normal password. Even that is still available is you use one of their secure mail keys. I suggest you look it up in Yahoo help.

If oAth does not work go to options and check that cookies are allowed. Some security software turns them off because you need to be protected from normal Internet things apparently. If cookies are off turn them on and try to authenticate again.