Reading some (but not all) of the Firefox/Thunderbird discussions, I find myself sanguine. I am not finding anything to be upset about. I think the current discussion may be more about the social structure of Mozilla. It does not seem to be a technical issue.
Looking at the technologies, it might be helpful if MoCo became, or came to see itself as, a service-provider for projects like Thunderbird. It could be "sourceforge with benefits", or perhaps a "sourceforge with a conscience". This seems obvious.
What makes it hard for MoCo to deal with Thunderbird as it is right now? Open-source avoids some problems one sees with commercial development. Commercial development seeks local maxima of utility with minimal investment of money. But MoCo has a similar issue. It seeks local maxima with minimal investment of social capital. What does this mean? If one looks at how Mozilla works, one sees this played out in many ways. I see some of them, but I do not think I am the best one to call them out.
One can identify, though, the "Mozilla way" of doing things. I noticed this when I was at Apple, and I still see it today. The "Mozilla way" is to use quirky tool sets, often old tools sets, with lots of hand-crafted modifications for particular issues. Enabling flexibility in these systems is not often a priority. Often it seems to be easier to hand-craft a solution to deal with the hand-crafted solution from two years ago. Finding a general solution that does not require tweaking does not seem to be a priority. A general solution rarely has social benefit. Individual tweaks, each to satisfy a different social entity, do have social benefits.
Why are more things not better documented? There is no social benefit to documenting that which is known by the core group. MoCo has, in general, a resistance to making things obvious. But social networks are built on shared knowledge, and if that knowledge requires a "maturation ritual", a "rite of passage", to find, all the better.
So, why can't Mozilla keep Thunderbird up-front, alongside Firefox? Perhaps it is not a technical issue. Anthropologists tell us that primates form social networks whose size varies with the ration of the brain size to body size. Given MoCo's reliance on social cohesion, Mozilla may not be able to concentrate on Thunderbird because the social network required is too big. Is Mozilla's pervasive reliance on social cohesion a good thing?