FireFox OS could have provided such model, in fact, its successor kaios is doing very well and one can imagine that in the future, it will be the primary OS of half the mobile users on the planet. 'Feature phones' aren't dead. They provide a cheap alternative to touch phones, are usually more robust, and allowing them to run web apps instead of MIDP stuff is a giant opportunity for any web actor.
Ditching Rust as a core component of the future of Firefox is also a demonstration that Mozilla isn't a tech focused corp anymore. Rust is going to yield a lot of result when it comes to security, memory saftely and maintainability and firing Rust devs was terribly short sighted.
So yes Firefox was always enough. Leadership at Mozilla doesn't get it.