Also, the generated JIT code isn't static and does not simply map source to native like in statically compiled languages. A lot of JS JITs, including Firefox'es

Also, the generated JIT code isn't static and does not simply map source to native like in statically compiled languages. A lot of JS JITs, including Firefox'es, dynamically rewrite the emitted code as it executes to add support for newly-seen types. So not only will the code be different environment to environment, it will also be different within the same script execution.