Thus one cannot make a JIT compiler for js and thus pages are slow.
Game engines such as Unity use il2cpp to go from the C# stuff into C++ that can be statically compiled as running the C# as bytecode would be too slow without a jitter.
The interface between SpiderMonkey and the rest of Firefox is here: https://searchfox.org/mozilla-central/source/js/src/jsapi.h.
Nitpick: you can but you cannot ship it via the App Store.
> Thus one cannot make a JIT compiler for js and thus pages are slow.
Lack of JIT doesn't slow down pages much. I believe the difference is so small that it might even be possible to be faster without JIT for most pages, especially on low power devices, it's only computation heavy pages that are going to suffer and even then it's not like you can't optimize common patterns statically ahead of time.
It doesn't mean that it's a good idea to invest into it, you are still under Apple's mercy and they clearly don't want competing browser engines.
So why do you think so many major companies invest such a huge amount of money in JS JITs? Google, Apple, Microsoft, Oracle, Mozilla all maintain JS JITs and spend what I guess is hundreds of millions of dollars a year on it.
