Review for Provider for Google Calendar by Tomas
Rated 5 out of 5 stars
One more note, maybe it will help:
If I select the item "Refresh Calendar" to "Manually" in the calendar settings, the synchronization works. If I choose "Every 30 minutes" or "Every 60 minutes", the sync doesn't work.
To create your own collections, you must have a Mozilla Add-ons account.