I run the sunspider test on most popular browsers last week and Firefox 3.1 beta2 was the fastest
with Chrome beta a very close second (and IE 7 like 20 times slower!!). The results were :
  • Firefox 3.1 beta2: 1997.6ms
  • Chrome : 2240.8ms
  • Firefox 3: 4293.4ms
  • Opera 10 alpha: 5088.4ms
  • Safari 3.2 : 5418.4ms
  • Opera 9.6 :5494.2ms
  • IE 7: 42412.8ms

Now the final version of Chrome does it in the blinding speed of 1551.0ms!!
I wonder what will be the limit for javascript speed in the future. Maybe Firefox 3.1 will push it a bit more down.

Your results may vary depending on PC specifications and operating system
PC specifications: Core 2 Duo 3.2 Ghz 2GB ram XP SP3 (Tested on a friend's PC, didn't use Linux in order to test Safari & IE)