Review for Provider for Google Calendar by Sam Wilson
Rated 5 out of 5 stars
The only thing I noticed lacking was some progress indicators while it was connecting to Google to retrieve e.g. the list of calendars. A white window looks odd, but it got there after 30 seconds or so, and everything's worked great since. Sync works in both directions.
This review is for a previous version of the add-on (3.3).To create your own collections, you must have a Mozilla Add-ons account.