Mozilla is moving its IonMonkey JIT much faster to Firefox than we anticipated.
IonMonkey will upgrade the current JaegerMonkey, which was introduced with Firefox 4 in March of 2011, and bring Mozilla closer to Google's Crankshaft that is used in Chrome. The progress of IonMonkey can be seen at arewefastyet.com, which shows that, at least on Mozilla's test platform, IonMonkey is faster than Google's Chrome in Mozilla's Kraken benchmark, but still has some catching up to do in Sunspider, and rivals Safari's score in Google V8 benchmark.
The IonMonkey Firefox can be downloaded from Mozilla's nightly channel now. The Aurora developer version is due on October 9, while the Beta channel is likely to see IonMonkey on November 20, if Mozilla can move IonMonkey through its developer process smoothly.