How do I configure Mozilla Thunderbird to work with Zimbra Calendar

Download the Lightening Extension -> http://www.mozilla.org/projects/calendar/lightning/

Install the add on -> Tools -> Add-ons -> Click the circle beside the search box -> Select Install Add-on from File

Restart Thunderbird

Click the new calendar icon in the top right.

Right click the whitespace on the left and click "New Calendar"

Select On the Network -> Continue

Select iCalendar (ICS) -> Type https://yourmailaddress.com/home/yourusename/Calendar

Type in your username/password and Your calendar should show up now.

Tested on:
OS X 10.8.4
Thunderbird 17.0.7
Lightening 19.1

Source:  Link