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CloseReview for Provider for Google Calendar by mhughson
Rated 5 out of 5 stars
Signed up for an account just to review this. Really awesome add-on. Does exactly what it says.
As @SukiLou said, check out http://bfish.xaedalus.net/2007/04/stay-in-sync-with-gcal-and-thunderbird/ for details on syncing up with Google Calendar.
The main thing that didn't seem obvious to me is that you need to create a "new calendar" in Thunderbird, and that is where you put in the Google Calendar info.
To create your own collections, you must have a Mozilla Add-ons account.