This page is not complete.
This content covers features introduced in Thunderbird 6
This page lists the changes in Thunderbird 6 that are relevant for developers. For changes that are relevant to end users, see http://support.mozillamessaging.com/kb/new-thunderbird-6/.
Thunderbird 6 is based on Gecko 6.0. The Firefox 6 page has details on the significant changes to Gecko.
Changes for Mozilla and add-on developers
For helpful tips on updating existing extensions for Thunderbird 5, see Updating extensions for Firefox 4. There are several key changes that break compatibility with existing add-ons, so be sure to read that article.