The new Mozilla's JS engine, IonMonkey, brought a lot of performance improvements, and it only landed in Firefox 18. So most probably v22 would be a lot faster than v17. But, if you could use v24 it would be even better, as the performance is improving from release to release, and v24 contains also OdinMonkey (asm.js) that you could use in some cases (for example for the AI, instead of moving code to C++, you could use asm.js so that the code would be a lot faster and there wouldn't be any performance penalty caused by JS-C++ calls).