The change is Thunderbird adapting to a change required by Yahoo.

There are a series of steps to narrow down your issue...

B1. With the password that failed with Thunderbird, can you log into the account(s) using the mail provider’s web interface (webmail)? (Note: you must use the account password, not an “app password” that was created by yahoo) B2. What are the full domain name(s) of all the affected account(s)? (for example yahoo.co.uk) B3. Are cookies enabled in Thunderbird at Settings > Privacy & Security > “Accept cookies from sites …” B4. Are you able to login using Thunderbird after you do the following

• Settings
• Config Editor
• find mailnews.oauth.usePrivateBrowser (screen shot below - you can paste it into the search field)
• click the two-headed arrow icon to toggle false to true

* restart Thunderbird (make sure it has fully shut down before starting it up)

• retry the login

C. If you were not successful after B4…

C1. Can you see the entire oauth dialog panel - no action buttons are missing? C2. No authentication dialogs are hidden behind what you currently see? C3. You don’t have the preference javascript.enabled set to false in Settings > Config editor? C4. Are you in the country in which you normally use the account? (rule out geoip) C5. (related to #4 above) Are you using a VPN? C6. Is the authentication method in Account Settings set to OAuth2? C7. Are you using MFA/2FA? (enabling multi-factor authentication / two-factor authentication in your mail provider settings might be blocking you) C8. Are you running a webserver on your computer? (could be anything listening on port 80 or 443) C9. If you are using Windows, does it help to start Windows in Safe Mode with networking enabled? See https://support.microsoft.com/en-us/windows/windows-startup-settings-1af6ec8c-4d4a-4b23-adb7-e76eef0b847f. If you feel the instructions are too complicated, printing them often helps.

If you are really stuck, sending us screen shots might help. (Attached are some screen shots of what a user of yahoo should see.)

Firstly, thank you for reponding quickly, a rarity in this day & age. The info supplied was most helpful, I now have 2 of my accounts back. This has also highlighted the fact to make sure one's backup email address is still current! Sadly, now that I have been able to get back via your help, I have found that I am caught in an endless loop to get my other emails released, as both old emails rely on a code from each other and one of them has been terminated! Again, thank you.