See https://support.mozilla.org/en-US/questions/1451034

Unfortunately, the key that gets created by changing the authentication method to Oauth (from "Normal password") somehow is removed from TB's saved password list when quitting from TB. So once it is re-launched, TB goes through the same process of passing the authentication through to the Microsoft login page. It really is quite ridiculous. If this doesn't get resolved, we won't be able to use TB anymore after the change by Microsoft.

FYI, Yahoo and Gmail email account don't suffer this issue, i.e., the key for Oauth authentication does stick (as it should) after restarting TB.

Do oyu have an antivirus scanning in your profile folder. Crete an exception for Thunderbird profile folder in your antivirus product and see if it is saved then.

I am seeing a lot of folk with setting that just do not get saved. But none who have replied creating an exception was not effective. I don't know if it works or they just don't reply or think it is to hard.

By default. My AV is not configured to exclude this folder, but I can see the logic in this as a cause. I have checked the AV logs and there aren’t any activities related to this as a cause however.

Over yesterday and today, I have again updated the authentication protocol etc as per the Microsoft recommendations. I have been quite methodical, even more so than before, in that I have been making sure the Oauth2 key was created in the TB password database for each outlook mail account that I updated and restarting after each of these changes and sometimes after bundle changes.

Also often checking the TB password database to see for those that have been updated, their Oauth2 key is still listed. Sometimes I would relaunch the TB a while after shutting it down. This has been working as expected so far so I thought great, but now I am starting to get the Microsoft authentication pop up or mail accounts that have had their Oauth2 key listed in the database several occasions after TB reboot.

Then checking the TB password database, I can see a few more mail accounts that have lost their Oauth2 key. I just restarted TB and checked the TB password database only to find that now MOST of the mail accounts have lost their Oauth2 key. Again, no issues with gmail and yahoo accounts.

Not sure if I can be persuaded to exclude this folder from being AV scanned as the next step as I don’t see this being the problem and also, having the AV disabled on this folder would need a period of 2 days minimum (from the above test) and I don’t feel this would be safe.

Unfortunately, I will now need to reset the authentication for each mail account from back Oauth2 (again) as otherwise, TB is unusable.

I hope this helps.