** You must have Thunderbird version 2.0 and above to run Calendar add-ons.
Done, your Thunderbird should be sync with Google Calendar by now.
Now, You will be prompt to enter your Google Calendar login, enter your login.
Now enter a name for your Calendar and choose your calendar Color and mark if you need the alarm for all your imported event and click “Next” -> “Finish”.
Create New Calendar window popup -> Choose “On the Network” -> Choose “Google Calendar” -> Paste your private xml link to the “Location” text box and click Next.
Go to Thunderbird -> File -> New -> Calendar.
Go to “Settings” at the top left corner -> Click on “Calendar” Tab -> Click on your calendar name -> Look for Private Adress and copy the url for XML (Right Click -> Copy Link Location).
ics file you just export out from your Thunderbird -> Choose your calendar and click “Import”
Now, login to your Google Calendar and go to the Left Side bar -> My Calendar -> Click on “Add” and Select “Import Calendar”.
Go to your Thunderbird Calendar view, click on File -> Export Calendar -> Select your Calendar -> enter a name for the export file and select iCalendar (.ics) at “Save as type:” If you already have your current Calendar running, you need to export your Calendar.
If you have no Calendar running, you can skip step 2, 3 and 4.
Download and Install Thunderbird plugin – Provider for Google Calendar.