CalDAV Google Calendar in Thunderbird

• Open Thunderbird File->New Calendar 
• Select On The Network 
• Select CalDAV 
• Set Location to: https://www.google.com/calendar/dav/SPECIAL-SAUCE-GOES-HERE/events 

The "special sauce" that takes the place of SPECIAL-SAUCE-GOES-HERE is found on your Google Calendar (gCal) account.

• Login to your gCal account. 
• Go To Settings 
• Click on the Calendars tab 
• Click on the name of the calendar you wish to add to Sunbird/Thunderbird 
• Look for the "Calendar Address" entry, next to the XML ICAL HTML buttons is a Calendar ID in parenthesis. 

It looks something like this for all but your primary calendar: nxu54xul23v3g...@group.calendar.google.com
For your primary calendar it will just be your gCal account login: <your-name-here>@gmail.com