To diagnose problems with Thunderbird, try one of the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Thank you Matt. I tried, but to no avail. Actually, it's getting worse... and I note that I'm not the only one who's having this problem. I can no longer access any of my email accounts. It varies from day to day. Yesterday, only 2 accounts constantly asked me for passwords. Both of which "failed", despite being the correct passwords. Today, all 8 accounts. I've checked by going directly to the account webpages... all is fine. Except that Google is coming back with "suspicious activity" on ALL of them. Hardly likely. Is Thunderbird sending out spurious data causing this apparent "suspicious activity"?

Oh, this is becoming crackers! Each time I fire up TB, different accounts fail. There's no pattern.

P.S. I've managed to cancel the mysterious down-vote, Matt. No idea what had happened. It wasn't I.

A little more information. I've just fired up Thunderbird and all 11 accounts failed. I closed Thunderbird. A few minutes later, I fired up Thunderbird again... and all 11 accounts were fine. This inconsistency is strange... yes?

This is a few minutes later, I fired up Thunderbird and all 11 accounts failed again. And, if I try to send a message, this message pane appears. If I go to MS website and access my mail, no problem at all.

Okay... totally strange! All is working perfectly today.

I see two bug reports that might be relevant. https://bugzilla.mozilla.org/show_bug.cgi?id=1528136 Particularly the discussion to the end of the topic. and https://bugzilla.mozilla.org/show_bug.cgi?id=1642483