Rated 4 out of 5 stars

After update to v52.9.1 I had to enable javascript in Thunderbird (not Firefox) to get the Gcal to prompt for login in order to sync.

Tools|Options|Advanced|General|Config.Editor
then type javascript.enabled at the top and make sure the option is true (grayed out as a default value)

This review is for a previous version of the add-on (4.4.1).