Searching for info about Thunderbird 78.0 in the version release notes provided a somewhat disappointing result:
"Add-on support: As of version 78.0, Thunderbird only supports MailExtensions. Your favorite add-ons may not have been updated for compatibility."
So, apparently until some future date, there will be no way to connect Google Calendar to Thunderbird in the manner we're used to. The Mozilla add-ons page offers a couple versions that make use of Thunderbird's ability to display a web page in a tab, so you can still view your calendar in this less-than-convenient manner, but I've found no way to attach Thunderbird to my calendar and my wife's calendar that way.
And that still provides no way to display calendar info in the "Today Pane" as before.