alt Hacker NewsSignal is already ostensibly private, available, and popular enough, and doesn't have ads... why compete?
IMO Mozilla should just double down on the browser and do everything they can to keep it as a lifeline for Free Software devices to be able to participate on the internet as first class citizens.
Signal intentionally made their messaging rely on a single, central point of failure, perfect for targeting by all sorts of criminals and governments. If Mozilla provides a Matrix server, I will seriously consider it.