Review for Provider for Google Calendar by Coeur-Noir
Rated 5 out of 5 stars
Right method for r/w access
(as explained here https://developers.google.com/google-apps/calendar/caldav/v2/guide#connecting_to_googles_caldav_server and there https://blog.mozilla.org/calendar/2013/09/google-is-changing-the-location-url-of-their-caldav-calendars/)
is to use CalDav, with such type address :
https://apidata.googleusercontent.com/caldav/v2/calid/events
where calid is the html calendar address of that shape :
a1z2e3r4t5y6u7i8o9%40group.calendar.google.com
(at least for the moment)
To create your own collections, you must have a Mozilla Add-ons account.