Rated 5 out of 5 stars

UPDATE Jan 28 2019:
If Thunderbird cookie settings are set to refuse cookies, you must add an exception for https://calendar.google.com ! A simple exception for the entire google.com domain does NOT work! Once I did this, the Open Google Calendar v1.4 extension works as promised: i.e., it opens my Google calendar account in a Thunderbird tab.
Original rating 1, with review below:
Doesn't work with Thunderbird 64 bit v.65.03b
returns error from Google:
"400. That’s an error from Google:

The requested URL was not found on this server. That’s all we know."
I will upgrade review when extension starts working.