[amaranth]

I suspect their sandbox doesn't allow code generation since they statically verify you aren't using instructions they can't protect against and that would break it. That means that while you could probably get a Firefox running, it'd be with a Javascript interpreter, not a JIT.

[boulos]

While I don't work on any of the related pieces, it should be noted that NaCL has dynamic "check this code" support precisely so you can JIT-compile code and execute it safely.