Mozilla chairman Mitchell Baker said that there will be no further innovation updates and no further interface development. Mozilla will be offering stability and security updates via an extended support release process, but that's about it. Baker justified the move with a certain lack of community effort to continue Thunderbird development:
"We've tried for years to build Thunderbird as a highly innovative offering, where it plays a role in moving modern Internet messaging to a more open, innovative space, and where there is a growing, more active contributor base. To date, we haven’t achieved this."
She noted that localization requires substantial resources and the need to support Thunderbird today ties up localization resources and slows down other Mozilla products. However, community participation is just one side of the story. Baker, along with Thunderbird managing director Jb Piacintino, hinted that the time for the standalone email client application had passed and that users would be flocking to other interfaces. "We have seen the rising popularity of Web-based forms of communications representing email alternatives to a desktop solution," Baker wrote. "Given this, focusing on stability for Thunderbird and driving innovation through other offerings seems a natural choice."
Cutting Thunderbird efforts seems to be merely a strategy of consolidating its resources as Mozilla will need every resource it has for the development and maintenance of the recently announced Firefox OS. Additionally, keeping the pace of Firefox development will be critical for the company as well.