I hope they fixed the bug where all indexed e-mails get reindexed and disappear from the top pane.

Lanikai is the first semi-stable release of Thunderbird to use Gecko 1.9.2, which is the same engine that Firefox 3.6 uses. The changes made from Thunderbird 3 to the 3.1 beta test version include Mac OS X 10.6 upgrade path from Thunderbird 2 improvements, fixes for autocomplete, tabs, activity manager, minor interface improvements and corrections, and a spate of stability and memory corrections.

via Thunderbird beta ‘Lanikai’ released | The Download Blog – Download.com.