Other sources of Thunderbird information
From MozillaZine Knowledge Base
Add-on Guides and github based repositories
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