Those helped Firefox 4 beta 7 to achieve three to five times better results on some of the most popular browser benchmarks (like Kraken, Sunspider, and V8). In real life that means faster start-up time, improved page-load speed and better performance of Web apps and games.
Does this sound good enough to make you jump into the unstable waters of a beta release? Follow the link then.
For the rest of you, I’m afraid, you’ll have to wait till next year for the final version.