Although the Firefox browser is the flagship product at the Mozilla Foundation, it's far from the only one. There's the Firefox operating system, Mozilla Composer, ChatZilla, Firefox for Mobile, and all of their various components. One of the more popular niche products is Thunderbird, the standalone email client, but it turns out that working on it is not something that the Mozilla higher-ups want to continue doing.
Mozilla doesn't want to stop Thunderbird in its tracks though, but instead is leaning toward spinning it off so that more developmental time can be put into the Firefox browser.
"Many inside of Mozilla, including an overwhelming majority of our leadership, feel the need to be laser-focused on activities like Firefox that can have an industry-wide impact," said Mozilla's chairperson, Mitchell Baker (via Ars)."With all due respect to Thunderbird and the Thunderbird community, we have been clear for years that we do not view Thunderbird as having this sort of potential."
It may be the Foundation is hoping for some other entity to buy Thunderbird, as Baker alluded to looking for the right "legal and financial home" for it.
Thunderbird is a client that has proven to be relatively popular as an alternative to some of the more mainstream clients, and it likely would leave a number of fans saddened if development halted completely. Whether Mozilla will be able to find a home for the software before it becomes too much of a drain on the Foundation is anyone's guess.
As it stands, Thunderbird will continue to receive basic support in the form of security updates and any simple changes also made to the Firefox browser (as they both use the same rendering technology) but beyond that it seems unlikely that much more will be added.