Update: This guide was written in 2007 and refers to Thunderbird 2.0. Thunderbird 3.1 is available which includes support for the Mac OS X Address Book without needing any additional configuration. The information below is retained for reference.
It’s now possible to integrate the Mac OS X address book with Thunderbird. Thunderbird has its own address book that it uses on all platforms, but OS X’s address book integrates with lots of other programs and so having it work with Thunderbird is very helpful.
The feature is on the Thunderbird trunk and will make it into Thunderbird 3.0, which is due out in 2008. But you can get it now in Thunderbird 2.0 if you’re willing to jump through a few small hoops. The instructions are on the MozillaZine forums – basically, you will need to download a custom unofficial build of Thunderbird 18.104.22.168 with the feature enabled, and then add three lines to your configuration file.
Once this is done, an ‘OSX’ folder will appear in the Thunderbird Address Book, which will have your contacts in it and your contact groups. You’ll also find that when typing email addresses in the Compose Message window, Thunderbird will auto-complete addresses from your address book.
It’s not full integration yet – the access to your OS X address book is currently read-only, so you can’t update your contacts from within Thunderbird. But it’s not a one-time import, so if you add new addresses to the OS X address book, they will be reflected in Thunderbird.
Judging by the comments on the tracking bug, some people have been waiting for this feature for a very long time. Though we have a little while to wait before it officially ships, the fact it’s now possible is great news for many people.