Review for Provider for Google Calendar by Shelku
Rated 4 out of 5 stars
I've been using this for years and love it!
Lately I've been having more trouble with it sometimes not showing anything from my calendar. I also can't add my Google Calendar directly because it doesn't let me click OK to finish creating it - I can only add it as ICS and then let Thunderbird upgrade it:
1) New Network Calendar
2) ** iCalendar (ICS) ** mode (with XML link from Google Calendar)
3) Restart Thunderbird
4) Click to upgrade the calendar
To create your own collections, you must have a Mozilla Add-ons account.