Comment 59 for bug 810622

This seems to work for me:

Adding Calendar Tab:
Ctrl+Shift+C
OR
Events and Tasks menu, then click on Calendar

Making Google the Default Calendar:
Once Thunderbird starts -> Go to the Calendar tab and Right Click on the Google Calendar which you want the events to be processed by
Click on Properties
Select the Email -(gmail in this case – the calendar you want to make as default for accepting invitations)
OK-> Close

Hope this helps