have you tried checking that our anti virus firewall is not blocking Thunderbird (very common) or that cookies are enabled (required for oAuth authentication)

hi, can you delete the cookie related to google mail inside the TB, & try to access gmail again ? if above steps do not work, then remove gmail & add the gmail mail account back again in TB, in that way, it will set a new-cookie for OAuth2 auth-method, as TB was upgraded, some detection mechanisms in mail-server side, may require a new cookie in client side.

@matt, @MattAuSupport Gmail was working in thunderbird for years on my current setup yes, I have tried disabling the firewall, and AV had no effect. Cookies are enabled. Bringing up in MS Mail I get the authentication prompt and can log in. I don't get it in Thunderbird.

TB's mail-account passwords and/or mail-access key/code are saved inside TB , delete that too, (and the mail.google.com , google.com , etc cookie(s).

please check this answer , goto SMK paragraph or search (by using Ctrl+F buttons) for "saved" word, to see instruction on deleting saved password.

Restart TB , and click on Inbox or Sent.

Thank you all for your assistance in resolving this. The issue turned out to be SSL filtering by ESET and was solved by following the directions at https://support.eset.com/en/kb3126-disable-ssl-filtering-in-eset-windows-products. What I don't understand is why when I disabled ESET protection it didn't resolve the issue. Nor do I know what changed on 10 days ago when my ESET license renewed, either in ESET or thunderbird to cause the issue.

Again thanks to everyone who assisted in trouble shooting this issue.

hmm, your "close socket connection" error was caused by ESET's interception & block. i should have thought/detected that, as something was closing connection. more info on AV/FW/SS, here.