Rated 2 out of 5 stars

I suggest using CalDAV format to add Google Calendars to Thunderbird. No plugin is necessary. The Location (URL) should look something like this:
https://apidata.googleusercontent.com/caldav/v2/###/events
where ### is replaced by the Calendar ID (including @group.calendar.google.com or @gmail.com or similar).

This plugin has been buggy recently (as noted in other reviews) and it's not necessary, unless you want to import a task list from Google Calendar. CalDAV works much better.

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