A couple years ago the development of Thunderbird was split off from Mozilla into its own separate organization, Mozilla Messaging. This did not work out as well as hoped. Case in point, future development after Thunderbird 3.0 has been rather lacking over the past couple years. Mozilla has decided to re-take control of Thunderbird and merge Mozilla Messaging into Mozilla Labs. Mozilla Labs is a specialty team that handles the development of experimental projects such as Personas, Jetpack, Sync and Test Pilot to name a few. David Ascher will continue to head the development of Thunderbird under the new Mozilla Labs team.
Source: ars technica