A must have feature for TB instead of having separate Sunbird App Rated 4 out of 5 stars

It seems that Mozilla and its users are going toward merging the calendar system to Thunderbird. This addon is their main effort for achieving this.
It can easily sync with Google Calendar and other web calendars by default. But there're still some works remaining like the ability to sync Lightning tasks with Google tasks.

This review is for a previous version of the add-on (1.9).