That doesn't actually rebut the point. After all, Firefox OS's browser doesn't include a HTML renderer and JavaScript engine written in JavaScript; it outsourc

That doesn't actually rebut the point. After all, Firefox OS's browser doesn't include a HTML renderer and JavaScript engine written in JavaScript; it outsources this functionality to a sandboxed mode of the underlying C++ engine. If you want to use a different engine, it has to be written in JavaScript, which is of course completely impractical for performance reasons.