Problem
You want to add the Google calendar tab in your thunderbird but you don’t want to use the standard google mail calendar, but the one setup for your domain.

Solution
Install the Google Calendar add on in Thunderbird:

Tools -> Addons -> Get Add-ons and then search for Calendar (Google Calendar Tab)

The when thunderbird is restarted go and change the default domain for logging in to the calender in:

Tools – Add-ons -> Extensions

and select the Preferences in the Google Calendar Tab.

from there select the ‘Google Apps for your domain’ and then change the URL into something like:

‘http://www.google.com/calendar/hosted/your_domain_hosted_with_google