Rated 3 out of 5 stars

Version 78.0.1 (just installed on a fresh new Thunderbird installation) :

Unlike what others say, it does NOT work with Thunderbird 78.6.0. More exactly;
- display the google agenda works
- adding a received invitation DOES NOT work : message "No writable calendars are configured for invitations"
- adding participants to a new meeting DOES NOT work, after the event is saved and closed they are not saved nor do they receive a notification.

It was the best google agenda provider but it is broken for months now since recent releases of thunderbird 78.

So I tried to use Provider for CALDAV, it is a little harder to setup since one must find the URL of the Google agenda, but it works.

The URL is https://apidata.googleusercontent.com/caldav/v2/yourID/events where 'yourID' is the calendar ID given by google in the agenda settings.

This review is for a previous version of the add-on (78.0.1).