According to an article in Mozillazine the Mozilla organization is shifting corporate support away from Thunderbird to focus more on Firefox. Thunderbird still has the right to utilize Mozilla's servers for hosting, builds, version control, and bugzilla but they will no longer be working on Thunderbird as a product. Thunderbird will likely return to it's roots of being an open source project hosted by Mozilla servers but worked on mostly by volunteers. I hope this doesn't negatively impact Thunderbird's stability and feature set. For more info read the Mozillazine article.