Keep in mind that the MacBook Pro I am using is a legacy machine from 2017 using files that were originally configured on a 2011 MacBook Air. - The solution was to start fresh. - I uninstalled Thunderbird, plus remove any associated Thunderbird files found in multiple folders that lay within the Library folders. The Library files can be accessed clicking on the GO option on your top menu bar rather than the usual Find option. - Instructions can be found in the following link - which is a portal trying to sell software. - https://nektony.com/how-to/uninstall-thunderbird - So, after a much more thorough 2nd uninstall and a "fresh" reinstall - I am now able to send emails from this @utexas.edu address that utilizes the gmail server. The smtp.office365.com SSO log in prompt is gone. It's all Google now. My former email address utilized the Office 365 server. - That's a long story cut short. For most users, if they are using fresh install on a clean platform, (i.e. new machine, SSD or hard drive) should be able to use Thunderbird with their Google email account - but the individual settings will have to be configured manually - and - the settings are not intuitive. Thunderbird's automatic config option can't do it.