do you have something like a password vault with some sort of internet security? I know Norton used to have such a thing and happily supplied wrong passwords from it. Completely bypassing the Thunderbird password manager.

Hi Matt Thanks for your suggestion, but no; while I do use Avast as my antivirus, no form of password vault or anything I can think would interfere, not even a "master" password for TB. Presume I am looking in the right place?

If the passwords showing or not showing in Thunderbird are incorrect and everything is working correctly then something else has control and the obvious place is Avast.

You say 'Avast' but there are various ones eg: Avast Premium Security, Avast One, a Free version

You may find that Avast Password Protection is enabled by default when you installed Avast. You can allow or deny an application access to your passwords, it is automatically added to the relevant list on the Blocked & Allowed apps settings screen.

So, Avast could have all your stored passwords including anything you stored in the browser eg: Firefox Passwords

Thanks for the suggestion

Only Avast One in use, which i didn't think had anything to do with passwords. This indeed reports that none of the browsers I use, Chrome, Edge & Firefox have passwords protected. Thunderbird is not separately mentioned.

Have tried opening, settings, passwords; with Avast shields disabled makes no difference.

Feel sure it has to be somewhere within TB, mystery continues.

A scan with ExtPassword! reveals the only location the correct password is stored on my machine is : C:\Users\[user name]\AppData\Roaming\Mozilla\Firefox\Profiles\bi9r47um.default-1544820106121\logins.json

Are Firefox and Thunderbird sharing passwords but TB is reporting an old entry?

Deleting the account and re entering corrects the entry.

Sure the mystery will continue ;-)