Well, this is not the inside story, but what is known publicly. I'm just copy pasting from the Wikipedia article on Mozilla Thunderbird [1] since it has all the

Well, this is not the inside story, but what is known publicly. I'm just copy pasting from the Wikipedia article on Mozilla Thunderbird [1] since it has all the details with citations. Basically, Mozilla believed that email was a solved problem that didn't need attention while Firefox OS and others needed more attention. The code sharing between Firefox and Thunderbird also made management of change more cumbersome (it's still difficult on the Thunderbird side now, though it's better than how it was a few years ago). > On July 6, 2012, Mozilla announced the company was dropping the priority of Thunderbird development because the continuous effort to extend Thunderbird's feature set was mostly fruitless. The new development model shifted to Mozilla offering only "Extended Support Releases", which deliver security and maintenance updates, while allowing the community to take over the development of new features. > On December 1, 2015, Mozilla Executive Chair Mitchell Baker announced in a company-wide memo that Thunderbird development needs to be uncoupled from Firefox. She referred to Thunderbird developers spending large efforts responding to changes to Mozilla technologies, while Firefox was paying a tax to support Thunderbird development. She also said that she does not believe Thunderbird has the potential for "industry-wide impact" that Firefox does. At the same time, it was announced that Mozilla Foundation will provide at least a temporary legal and financial home for the Thunderbird project. > Since the memo in 2015, Mozilla has brought Thunderbird back in-house in an announcement on May 9, 2017, and continues to support its development. The Thunderbird development team has also expanded by adding several new members and has undergone an overhaul on security and user-interface. You could also take a look at the Thunderbird Council email archives (requires a login) [2] and the Thunderbird Engineering Council email archives (high posting volume). [3] P.S.: I donate money to the Thunderbird project. Anyone who wishes to help out could send a little money solely dedicated to Thunderbird. [4] [1]: https://en.wikipedia.org/wiki/Mozilla_Thunderbird https://en.wikipedia.org/wiki/Mozilla_Thunderbird [2]: https://mail.mozilla.org/listinfo/thunderbird-council https://mail.mozilla.org/listinfo/thunderbird-council [3]: https://lists.thunderbird.net/mailman/listinfo/maildev_lists.thunderbird.net https://lists.thunderbird.net/mailman/listinfo/maildev_lists... [4]: https://donate.mozilla.org/en-US/thunderbird/ https://donate.mozilla.org/en-US/thunderbird/