Rated 5 out of 5 stars

per darrask el set. 21, 2017 · enllaç permanent

This is indeed a simple but very useful add-on if you want Google calendar in Thunderbird. Works seamlessly, but needs internet.

Works in Thunderbird 52.3 despite the warning on the home page. Someone should update that plugin.

For Ubuntu, this will help you to make it work:
https://askubuntu.com/questions/404481/thunderbird-google-calendar-tab-acts-as-mobile-browser