Thunderbird is right now one of the two applications Mozilla is working on. The other one is Firefox. One can’t deny that the latter has gotten a lot more attention lately and now it seems things have gotten so skewed that Mozilla is seriously considering to move Thunderbird “to a new, separate organizational setting.” The reason is simply that Mozilla wants to focus more on Firefox. There are a number of possible scenarios; make Thunderbird the center of a new organization aside from Mozilla, make it a subsidiary of Mozilla or make it a community project, like SeaMonkey. The last seems like the most likely as it is also the one most supported by the current developers. We will have to wait and see what happens to Thunderbird, but right now it seems that Mozilla can’t do both Firefox and Thunderbird.