Download Thunderbird from Mozilla website:
https://www.thunderbird.net/en-US/download/
Double-click to mount disk image:
Ignore the arrow pointing to the computer's top level /Applications folder and instead drag to your own personal Home folder:
Launch Thunderbird and enter just your name and email address; leave password field BLANK
Confirm settings as below and click "Done"
Sign into Google with your email address
Sign in with CalNet Authentication and Duo 2-Step
Done!