Comment 2 for bug 1850651

It's a known bug in Thunderbird. It's fixed in the latest 68.2.1 release: see

In the meantime, until a version of Thunderbird with the fix is available for Ubuntu, there's a Mozilla support thread: that in turn points to this GitHub issue: (whew!) that finally (whew!) suggests:

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