Other sources of Thunderbird information
Add-on Guides and github based repositories
Mailing list based discussions
The mailing lists have moved from using the Mailman list manager to https://thunderbird.topicbox.com/groups . Topicbox's slogan is "Group email for teams". It works like a mailing list, and by default subscribers receive emails of all posts. It also has a decent web user interface.
There are currently groups for Add-on Developers, Thunderbird Announcements, Thunderbird Beta, Thunderbird Daily, Thunderbird Developers, Thunderbird e2ee, Thunderbird Enterprise, Thunderbird Planning, Thunderbird Support Crew (not public) and Thunderbird UX.
The tb-election and thunderbird-drivers lists are an exception. They were semi-private so they will be moved to private google groups rather than Topicbox.
The project stopped pointing users to newsgroups such as mozilla.support.thunderbird newsgroup in 2009. It appears all of the Thunderbird newsgroups will be shutdown.
Thunderbird is a volunteer project involving many moving parts, people and issues, so it does not have a fixed, dated release schedule. Releases occur at some point after Firefox releases, on a best effort basis. When you see it mentioned in release notes and in automatic update then you can be reasonably certain it is officially released - other sources of information generally cannot be trusted to be accurate.
Most of these specifications were written for Thunderbird 3.x or earlier.
https://developer.mozilla.org/Special:Tags?tag=Thunderbird lists all of the pages tagged with Thunderbird on the MDC web site