well what do you want to know?

That is what is the context. Are you asking how to become a developer, information of the developer network or are you asking who develops Thunderbird. At this point I am confused.

I was mostly looking for information on the schedule and features for upcoming versions, but also curious where to find things like the source code and instructions for helping Thunderbird with code plus other work.

Presumably there is a place where there developers discuss and plan development. All the links to these types of places on the Thunderbird homepage were the general "Mozilla" pages. Is there a reason it doesn't link to the Thunderbird-specific stuff? How do potential volunteer developers explorer Thunderbird?

Thunderbird is a community project and while Mozilla's involvement is significantly reduced, that does not get the code base out on it's own. Thunderbird and SeaMonkey are basically the comm central folder in the Mozilla code base. Hence everything linking to Mozilla. Thunderbird shared the Geko layout engine with Firefox and to all intents and purposes that is almost indistinguishable from Firefox. The result is Thunderbird developers find themselves working on core to fix Thunderbird bugs, even though it is technically part of Firefox.

Everything is controlled through bugzilla. enhancements, faults, poor usability and design. You do not need to do more that assign your self a bug and start coding. Although the involvement of the module owner or a peer helps get patches accepted. https://wiki.mozilla.org/Modules/Thunderbird

Bugzilla is used to extract details of bugs fixed (including enhancement) for the documentation of release notes. Because flagging bug for inclusion in release notes is manual there are always those the just appear without a release notes.

Talking or communicating with other developers appears to still be mostly IRC. (Global means Global at Mozilla with people in many countries collaborating)

This wiki page https://wiki.mozilla.org/IRC after lots of instructions does tell you what channels are for what in Mozilla IRC.

Links to mercurial, are on this page. https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Source_Code/Mercurial

Downloadable source here https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/Source_Code/Downloading_Source_Archives

Is there a specific area you were looking at? perhaps I can point you to someone.