The nag as you so blandly describe it is a request to log in. I have no idea what choices you made when your set up the account in Thunderbird, so it could be something to do with contact sync or even calendar sync depending on what addons you have installed. Actually completing it, which would initiate an oauth2.0 flow will disclose exactly what the request is to authorize, so try that and post back when you know what it is you are attempting to ignore.

Let me try to describe this in more detail.

One of my e-mail accounts is POP3 and configured to communicate with a mail server at outlook.office365.com.

I start up Thunderbird and retreive/send e-mails from/to this account. No issues.

Then, some minutes later, the troublesome behavior starts (and this behavior did not occur prior to some recent Thunderbird update). The behavior is

Thunderbird starts a new process (shows up in task manager), contacts the mail server, spawns a top-level window, and displays a login dialog. (I base this conclusion on the fact that if I use task manager to kill that process, the window disappears, and Thunderbird continues to operate properly, capable of sending/receiving e-mails on that account)

Since I gain nothing by logging in (and I have no Thunderbird add-ins), and since it was a struggle to get live.com out of my laptop, I do not log in, and do not plan to. I close window with the log-in screen, but minutes later it reappears.

It is a nuisance to have to keep closing this window, so I would like to stop this behavior.

Jonathan