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.
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.