Mozilla does have a Calendar application, called Sunbird, but it is a stand-alone program. Not to worry though – Mozilla realised that having a calendar built in to Thunderbird would be useful, so they concurrently developed an Add-On to Thunderbird that effectively merges the two programs together.

Thunder & Lightning

Aptly named “Lightning,” this Add-On to Thunderbird adds a sidebar listing your upcoming schedule in addition to a full tab showing your entire calendar. Adding events to the schedule is easy to do. In fact, Lightning is very effective at managing a number of calendars at once: you can manage your personal schedule, work schedules, and group schedules easily.

Sync with Google

The powerful Add-On, “Provider,” opens up even more organizational options, as it allows you to download and sync your Google Calendar into Thunderbird. Using  Provider, you can easily create events in Thunderbird and then have them automatically sync with your Google Calendar. This combined application lets you sync your calendar between your home and work computers, your laptop, your smartphone, iPhone/iPod Touch, iPad, or friends and co-workers.

How-To Step-by-Step

[Editors Note - The following steps apply as of 04/04/2010.  For up-to-date technical details, reference Google's limited support page on this subject.]

1. If you haven’t already, download and install Thunderbird
2. Download Mozilla Lightning to your computer
3. Download the Provider for Google Calendar extension to your computer.
4. Launch Thunderbird, go to the Tools drop-down menu and select “Add-Ons”
5. Click the “Install…” button, find a file you just downloaded, and hit Open
6. The extension will install. Now, repeat again “Install…” for the other extension you downloaded.
7. Click the “Restart Thunderbird” button
8. To sync your Google Calendar, continue, otherwise you’re done!
9. Open your web browser, log in to your Google Calendar
10. Click on the “Settings” link under “My calendars”
11. Go into the settings of the calendar you wish to share
12. Scroll down to “Private Address:” and click the XML link and copy the address it gives (select the text, go to the Edit menu, select Copy)
13. In Thunderbird, go to File menu, and under “New >” find “Calendar…” and click it.
14. Select “On a Network”, and continue
15. Select “Google Calendar”, and paste in the text copied earlier. (Edit menu, Paste)
16. You’ll be asked for your Google Calendar login and password (your Google account email address will be the login)
17. Choose a name and color for the calendar, and you’re ready!