That's for Tracemonkey, the new, somewhat faster javascript engine that's included with Firefox 3.5. Don't be alarmed then when Firefox 3.5 performs only marginally faster on javascript heavy sites than Firefox 3.0. No 64bit support for the new javascript engine, on the platform where Firefox pretty much dominates. In my view, that should've been Mozilla's first priority on Linux: many linux users choose 64bit because they are not hindered by missing drivers or applications. Linux users are typically more computer-savvy than your average windows or OS X users and could serve as good guinea pigs for a 64bit version of Tracemonkey. In any case, irrespectively of operating system, Tracemonkey 'feels' slower than Nitro (or v8) in 32bits too, but giving 64bit linux such a low priority is somewhat dumb, especially given the ever increasing interest and market share for 64bit linux among linux users (after years of requests Adobe released its exceptionally stable 64bit flash player for linux a few months ago). Hopefully a 64bit version of tracemonkey will be included in one of the minor upgrades before 2010, but I feel like Mozilla has made a mistake here.
Update: It seems that there is no 64bit Tracemonkey code, linux or otherwise, which is surprising; I am not exactly clear as to what prevents them from releasing a 64bit version.
The people behind the best HTML engine around (yep, that's WebKit for those that don't know it yet) just announced the next generation of their new Javascript engine, SquirrelFish Extreme, just a few months after SquirrelFish itself was announced and before it was even adopted in any product. In the performance chart they posted in the linked page the engine is around 3 times faster than JavascriptCore and two times faster than SquirrelFish. Impressive.
