Ah, so it is all about the javascript engine? Ok I can see how that might put up some major technical barriers, but still it should be technically feasible to have the firefox rendering engine make use of the iOS javascript engine, no? Maybe it's not worth the effort of course. I am just trying to get a feel for what the barriers are.
This is not at all the code to embed SpiderMonkey in Firefox, those are the bindings from DOM APIs to the implementation of those APIs in Firefox. Above that, calling into this layer, is another layer that transforms JS calls into C++ calls (roughly).