From MozillaZine Knowledge Base
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Thunderbird is a volunteer project involving many moving parts, people and issues, so it does not have a fixed, dated release schedule. Releases occur as 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 reasonable certain it is officially released - other sources of information generally cannot be trusted to be accurate.
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https://developer.mozilla.org/Special:Tags?tag=Thunderbird lists all of the pages tagged with Thunderbird on the MDC web site