Comment 2 for bug 1850651

It's a known bug in Thunderbird. It's fixed in the latest 68.2.1 release: see https://www.thunderbird.net/en-US/thunderbird/68.2.1/releasenotes/

In the meantime, until a version of Thunderbird with the fix is available for Ubuntu, there's a Mozilla support thread: https://support.mozilla.org/en-US/questions/1271710 that in turn points to this GitHub issue: https://github.com/kewisch/gdata-provider/issues/26#issuecomment-547994463 (whew!) that finally (whew!) suggests:

"Changing "general.useragent.compatMode.firefox" preference to true fixes the issue, google authentication works again."