The existence of this intermediate representation of the code and the optimizations done to it result in some big improvements over the previous JIT.
when IonMonkey was first enabled in Firefox 18.
"But also, it’s been a highly focused, year-long project on behalf of the IonMonkey team, and we’re super excited to see it land," he added.
IonMonkey was made the default when Firefox 18 was in the Nightly channel, you can read our coverage with more details here
It's better than what the initial benchmarks would indicate, the current IonMonkey serves as the basis for what's to come, the technology will be improved and optimized.
Starting with a powerful engine means Firefox will get faster and faster for some time to come.
In fact, IonMonkey is easy to improve by virtue of its design, the team can implement new or different optimization algorithms without any changes to IonMonkey itself. This freedom for experimentation should lead to much bigger performance improvements than before.