after a discussion with @JHarp, i also found that thunderbird 60.9.0 was refusing to accept my gmail account and password. according to this it was a change in google’s oauth process. the stated fix was:
Changing “general.useragent.compatMode.firefox” preference to true fixes the issue, google authentication works again.
i originally found that github link from a thread on the Thunderbid subreddit. i was unsure how to navigate to that particular preference, but KrillDerelictLeeward’s kind reply was enough to get me started in the right direction.
at the top right of thunderbird i clicked on the three lines. in the right-hand column i went to Preferences and then Preferences again in the menu that popped up. in the Thunderbird Preferences window, there is an Advanced tab at the top. under the Advanced tab there is a General tab. at the bottom of the General tab there is a Config Editor… button. after clicking that and accepting the risk(!) of voiding my warranty, i typed “useragent” (without quotes) into the search box. i double-clicked on the value field of general.useragent.compatMode.firefox to change it from false to true and then i was able to add my gmail to thunderbird.
for those who are a bit more visually oriented like me:
three lines (menu) -> Preferences -> Preferences -> Advanced tab -> General tab -> Config Editor… -> accept risk(!) -> useragent search -> double-click value field of general.useragent.compatMode.firefox for true.