FWIW, people in Mozilla are working on a "JIT coach", which will tell you if performance critical sections of your code aren't getting JITed, and why. I believe this is almost ready for use, though I'm not sure when it will be presented.
Sounds analogous to the way the chrome profiler tells you when functions have permanently deoptimized, right? That would be a terrific feature for Firefox, looking forward to it!
