Review for Provider for Google Calendar by Andy Granger
Rated 5 out of 5 stars
Fantastic add-on.
One issue we're seeing at my organization that I haven't seen mentioned by any other commenter: So far, everyone's events are showing up just fine, but no one can dismiss any reminders.
I don't know if it's relevant, but my company uses Google Apps for Work.
To create your own collections, you must have a Mozilla Add-ons account.