Review for Provider for Google Calendar by ytifle
Rated 5 out of 5 stars
You could also use CalDAV to sync the calendar with google calendars.
https://blog.mozilla.org/calendar/2013/09/google-is-changing-the-location-url-of-their-caldav-calendars/
Just follow that instructions, login, and it's working. (Bidirectional sync, not just for downloading, but be sure to pick the right calendar when creating events).
To create your own collections, you must have a Mozilla Add-ons account.