Because Dissenter was created by Gab, a far/alt-right online platform. Such groups are known for fostering hostile environments online towards people of various religions, races, cultural/political views, etc.. It is by no means a stretch of the imagination for Mozilla to claim that users of the app were using it for such purposes. As such, it would be apropos for Mozilla to remove an app that provides a platform for individuals to disrespect others.
Were Mozilla to keep it up, they would not be "respect[ing] their users".
Were Mozilla to keep it up, they would not be "respect[ing] their users".