Google Calendar is a great, if not the best available tool to share calendars. However, it’s tedious to edit and keep track of it via the web interface. In saving time, integration is your best friend.

Naturally, there is a vast amount of addons and tools to sync, integrate and simply work more efficiently with Google Calendar. For example you can sync it with mobile devices like the Blackberry or iPhone using GoogleSync. Then you can add Google Calendar to Gmail with several different Greasemonkey scripts. Now I will explain how to add it to Thunderbird.

For this you will need two Thunderbird addons:
1. Lightning adds a Sunbird type calendar to Thunderbird.
2. Provider for Google Calendar connects Lightning with Google Calendar.

After installing both of these plugins, you will notice several changes in Thunderbird. First of all there is a new sidebar holding the calendar. At the bottom of the old sidebar that is hosting the folders and address book, you should see three new buttons: Mail, Calendar and Tasks.

To integrate an existing Google Calendar, you first need to fetch your calendar’s address. Head over to the web interface, click the little arrow next to one of your calendars, and select >Calendar settings from the menu.

In the Details window you’re looking for the Calendar / Private Address section at the bottom. I found that both XML and ICAL URLs work, but XML is recommended. Whether you pick the calendar or the private address depends on whether your calendar is set to public or private.

Back in Thunderbird the first thing you have to do is click the Calendar symbol to switch from Mail to Calendar view.
Then click >File >New >Calendar… If the Calendar option is not available, you probably forgot to switch to the Calendar view.

From the Create New Calendar window select >On the Network, click >Next, select >Google Calendar as the format and insert the URL into the >Location field, click >Next, eventually enter your Google >User Name and >Password and click >OK, >Name your calendar, give it a >Color, click >Next and finally >Finish to complete the procedure.

And there you go, you have integrated your Google Calendar into Thunderbird. Editing the calendar works bidirectional, meaning you can add and edit events via the website or in Thunderbird and they will be synced automatically.

The Mozilla Wiki has a thorough page covering Provider of Google Calendar, including a section about bugs and limitations. There also is a Google Group discussing further questions.