Hours ago, a "shocking" Mozilla announcement has been published (more or less intended), mainly stating "Mozilla is focusing a lot of its efforts towards important web and mobile projects, while Thunderbird remains a pure desktop only email client. We have come to the conclusion that continued innovation on Thunderbird is not a priority for Mozilla and that the most critical needs for the product are on-going security and stability. In fact, it is quite possible that Thunderbird is already pretty much what its users want and there is not a high demand for innovation in this field".
The shocking aspect keeps its strength only at a first glance, yet, after careful examination, it seems natural (especially in open-source communities): Mozilla is not closing/ditching Thunderbird, but offers only security and stability maintenance, while, if communities are interested, being opened for unlimited ongoing development (but not with Mozilla's energy).
For Ubuntu users, this attitude is neither new nor "scary", we have all been witnessed successful project risen from the ground of organizations, such as LibreOffice that, after departing from Oracle, has been gathered numerous contributors (hundreds at the moment) and pushed into the office suite real innovation, exciting public contests (various CALL FOR FEEDBACK, CALL FOR VOTING, etc) and fruitful openness towards users interested in it.
A main aspect, tremendously important, is that Mozilla is to further offer (remain to be seen the amount of time) professional support, such as under-the-hood code stability, meaning, the area where full-fledged experienced engineers are needed, while opening for innovation the area where not-so-skilled coders can "interfere" (such as user interface, overall polish, new features, etc), making the "leaving approach" not a loss but a potential gold mine.
While step-by-step investigating, the user can observe a crucial fact in not rejecting Mozilla's Thunderbird approach: Thunderbird has already been, in the last months (years), enriched with mainly security and stability improvements, adding minor changes (in Ubuntu, new monochrome icons, tabs on top, option to choose payed email accounts, option to use payed storage, etc), consequently, the "shocking" Mozilla attitude has (almost) already been in place for a solid amount of time, probably this at-the-moment statement it is merely an official recognition.
At the moment, the are available two versions of Thunderbird, Thunderbird ESR (Extended Support Release) and Thunderbird, "Thunderbird will be released with the same feature set as Thunderbird ESR and will be updated every six-weeks for security and stability. However, and contrary to Thunderbird ESR, Thunderbird feature set might evolve over time and solely based on the availability of community contributions".