In the administrative part of that post, Mitchell announced that the Mozilla Foundation under Mark Surman has been working with Thunderbird to provide at least a temporary legal and financial home for the Thunderbird project (which we have been sorely lacking for several years). At the same time, a formal process will be undertaken to determine what is the best long-term home for Thunderbird, which might be Mozilla or might be some other entity.
Practically what this means is that in 2016, Thunderbird will finally be able to accept donations from users directed toward the update and maintenance of Thunderbird. In the long run, Thunderbird needs to rely on our users for support, and not expect to be subsidized by revenue from Firefox. We welcome this help from the Mozilla Foundation in moving toward our goal of developing independent sources of income for Thunderbird.
In the technical part of that post, Mitchell reiterated that Mozilla needs to be laser-focused on Firefox, and that the burden this places on Thunderbird (as well as the burden that Thunderbird places on Firefox) is leading to unacceptable outcomes for both projects. The most immediate need is for the Thunderbird release infrastructure to be independent of that used by Firefox, and Mozilla has offered to help. In the long-term, there will be additional technical separation between Firefox and Thunderbird as a continuation of a process that has been ongoing for the last three years.