Try putting a different browser renderer on Firefox OS. You're pretty much locked in to JavaScript — unless you can somehow build WebKit with Emscripten, you're already behind what iOS can do — even with Apple's rules.
In Firefox OS, the browser is the OS. It'd be like saying you can't replace Dalvik in Android.
It's not about replacing the browser, it's about being able to replace things that you should be able to. There is no good technical reason to not let people replace Safari on iOS.
Not so sure about your last sentence. Can a replacement browser be any good without using JIT compilation somewhere? Is the sandbox going to allow it to jump into dynamically generated data? Are there any valid technical reasons for having sand-boxing in the first place?
