Mozilla has released Thunderbird 19 Beta 1 on January 18, 2013. Note: Developmental (Beta, Earlybird and Daily) builds will continue to follow the current Gecko version numbering. This release includes fixes and improvements to chat.
Lightning Users: Lightning version 2.1beta1 is needed for Thunderbird 19, however this version is not currently showing on the Development Channel section of the Lightning page at AMO.
Thunderbird 19 will be released as Thunderbird 17.0.3 (or the current Thunderbird ESR version) on February 19, 2013.