The Mozilla Manifesto says nothing about web browsers. If you think they should throw it out and focus on making web browsers (and abandon their mission as an organization) you'll have to take it up with them I guess.
That's why they've tried in the past (and hopefully continue to do so) to provide resources like you're pointing at to "the public" to build and create their own special place on the Internet. Something outside the gated ecosystems of those big internet companies (Facebook, Twitter, etc).
Have they gotten it perfectly right every time? No, but that's okay, and it's good that they're even trying because not that many people (or companies) are.