As David Ascher has been hired  to start a company code-named MailCo, David Bienvenue and Scott MacGregor have announced their departure from the Mozilla Foundation umbrella, while committing to Thunderbird development as they retain their module owner hats.
Some of the Thunderbird users (particularly here in Europe, where Thunderbird has a lot of success) may be interested in knowing more about the future of their favorite email client.
A few days ago, David Ascher wrote an Open letter to the Thunderbird community. Today, Mitchell Baker just published Thunderbird Process of Change Part 1 (another post will get into more details). Both blog posts demonstrate that Mozilla is deeply committed in making Thunderbird an even more successful product. Three million dollars are going to be invested in this venture (that's 1.5 times more than what Mozilla Foundation has received from AOL for Mozilla Suite, Firefox and Thunderbird!), and I'm confident that Thunderbird has a bright future.
Update: Mitchell has just posted on her blog the second part of her article: Thunderbird process of change, part 2. Small excerpt:
(...) Thunderbird developers came up with the suggestion that they create an independent company and we move Thunderbird development to it. The company would be interested in promoting the Mozilla mission, but would be outside of the Mozilla umbrella of organizations; a private company owned and run by the developers. (...) This option had the advantage of exciting the developers. When we considered the idea of Thunderbird moving to a private independent company, a number of significant disadvantages emerged. First, this would mean that Mozilla as an organization was leaving the mail/ communications space and hoping that space would be filled by another organization. We weren't ready to do that. Second, it felt like this would be moving Thunderbird to a more private space. We're eager to see Thunderbird become an even more public project, with more contributors with greater authority. This concern is not remotely a reflection on the motivations of Scott and David. They have been devoted Mozilla participants for many years. This is a *structural* concern. It reflects the desire that Thunderbird - the product as well as the code - remain dedicated to the public benefit through the Foundation.
 Even if Thunderbird had less success than Firefox, several million users is nothing to sneeze at! If you want to understand better how Firefox and Thunderbird differ, you may want to read Seven things that make Firefox and Thunderbird different animals.