See https://support.mozilla.org/kb/microsoft-oauth-authentication-and-thunderbird-202

Hi christ1,

I have every confidence, that the instructions in your link are Valid :-)

However, I am just a simple user of emails / calendar, and was quite happy with the old Windows Mail / Calendar, until they forced everyone on to the New Outlook :-(

Thunderbird was an excellent alternative, until this issue arose, and all the solutions are so complicated, that I have Given up, and am seeking an easy free alternative ?

Can you login to your account via webmail?

Please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button ≡, then select Help > More Troubleshooting Information. Press the "Copy text to clipboard button" and paste the information into your reply.

I have already uninstalled Thunderbird, and am (so far) happy with an alternative provider, but Thank you for your support :-)