A total rewrite of Firefox for Android with the new GeckoView engine is going live soon. We have close to 1,000 engineers doing impactful work on things like engine development, Rust, WebAssembly, AV1, etc. Our designers are busy because design really matters to getting product into hands!
Except creating a fast browser that obeys web standards. I switched to an old fork of Firefox that's fast and now in maintenance mode. Works fantastic.
GeckoView is substantially faster than the old Gecko mobile engine, just like Quantum/Servo did for desktop. I don't know what web standards you think are missing.