Ok, found the solution. To clarify, the VAST majority of the so-called "answers" are just absurd nonsense. Symptom - attempt to login to gmail and download/send email and Thunderbird offers popup saying can't login. Then another popup appears saying to enter your Google password. You enter your Google password and it's not recognized by Thunderbird and the same popup asking for password appears. Infuriating circle of frustration, as you are indeed entering the correct Google password. HOWEVER, the password you need to enter is a special one-time password that's different than your main Google password. So you need to generate that one-time special password and enter it into that infuriating popup in Thunderbird. Solution: Log in to your Google Account. Select My Account > Sign-in and Security > App Passwords > Select the app and devices you want to generate password for > Generate. Then copy/paste this one-time password into that infuriating popup box in Thunderbird (mine was 16 character p/w automatically generated by Google - I presume everyone else's will be too) This will generate that elusive password you need to allow Thunderbird to access your gmail again and work like normal (it's a one-time password, so don't have to remember it). Have to say, most people who respond to these questions should be lined up against the wall - seems like every cockamamie, half-baked, inappropriate "answer" is offered. And it's typically cryptic, incomplete and generally worthless.