I'm not particularly technical, but here's what this all looks like to me.
The end result of this first round was that Firefox was really-really hyper-fast at a few things and behind somewhat on many other things. Now, with JagerMonkey, Mozilla is adding a Method JIT like Nitro and V8 have so we'll be well optimized for a majority of cases while preserving the Tracing approach where we can be super-duper fast when it's applicable.
But there's even more cool news. Once we get the method JIT in and we're on solid footing with all of the other JS engines for those common cases, there's still a lot of opportunity to improve our Tracing feature to cover more and more cases. So hopefully we'll see increases in the places where we can be out of this world fast, while making sure there are no places that are slow. That's enough to make you wonder if Apple or Google (or Opera) will decide they need Tracing in their JS engines. If they do, I hope they take a look at Mozilla's code so they can benefit from our work.