IIRC 5 years ago mozilla announced that thunderbird was not a priority anymore and considered dropping it.
Then 1 or 2 years ago mozilla announced that thunderbird sharing the firefox codebase was unnecessarily taxing firefox and the remaining thunderbird developers were struggling to follow the pace of firefox. Moreover that they believed thunderbird had little potential to have an impact as firefox has. So they proposed to separate thunderbird from firefox and cut the ties entirely.
Last news I heard was that Thunderbird could join seamonkey and become a community effort with some backing from mozilla[1].
Then 1 or 2 years ago mozilla announced that thunderbird sharing the firefox codebase was unnecessarily taxing firefox and the remaining thunderbird developers were struggling to follow the pace of firefox. Moreover that they believed thunderbird had little potential to have an impact as firefox has. So they proposed to separate thunderbird from firefox and cut the ties entirely.
Last news I heard was that Thunderbird could join seamonkey and become a community effort with some backing from mozilla[1].
[1]: http://www.ghacks.net/2017/03/10/seamonkey-thunderbird-to-jo...