By how much? The engineers responsible for bringing OdinMonkey into Firefox - love that name by the way - say that performance will be about two times slower than the performance of C and C++ native code.
You can check out the - technical - presentation here on Github. It includes the benchmark, some code examples and lots of information that should be interesting to developers.
You can also check out the Mozilla Wiki page where OdinMonkey is described on for additional information, goals and things that need to be done. (via Sören)