All of a sudden, Google (In this case, Gmail) disabled access to Thunderbird, identifying it as a "Less secure app). This happened a few days ago on December 11. I've been using Thunderbird for years configured with one of my Gmail accounts and have never seen this. Google blocked the sign-on attempts and sent me an email to that effect. I have been able to remedy the problem, having figured out how to turn on access to third party apps, but what a giant pain in the butt. By default, access to third party apps is turned off in Google accounts.
I like the way Thunderbird displays emails much more than the way Gmail does, so I'm going to continue to use it. Google is still identifying that there is a "Security risk" using Thunderbird, but this seems a bit heavy-handed to me.
Chris