Rated 4 out of 5 stars

I used this addon for a long time and it recently added task synchronization support too. But now it doesn't allow me to directly add my google calendar using "New Calendar -> On the Network -> Google Calendar", because it conflicts with the other addon "Thunderbirthday"!
There are 2 solutions:
1. Disable thunderbirthday, Add your calendars, Reenable Thunderbirthday when finished.
or
2.What Shelku said:
2.1. New Network Calendar
2.2. Format = iCalendar (ICS), Location = Private address in ICAL format (NOT XML FORMAT)
2.3. Restart TB
2.4. This addon will automatically detect your readonly ICS calendar and will ask if you want to upgrade it to a r/w google calendar

This review is for a previous version of the add-on (1.0.2).  This user has a previous review of this add-on.