Mozilla and Thunderbird are continuing together, with conditions
The Thunderbird e-mail client still has its supporters, but for the past couple of years, Mozilla has been making moves to distance itself from the project. In late 2015, Mozilla announced that it would be looking for a new home for Thunderbird, calling its continued maintenance "a tax" on Firefox development.
Yesterday, Mozilla Senior Add-ons Technical Editor Philipp Kewisch announced Mozilla's future plans for Thunderbird—the Mozilla Foundation will "continue as Thunderbird’s legal, fiscal, and cultural home," but on the condition that the Thunderbird Council maintains a good working relationship with Mozilla leadership and that Thunderbird works to reduce its "operational and technical" reliance on Mozilla.