Mozilla Links has an article about the Firefox ultimate feature - Performance, in which they compare the JavaScript performance on different web browsers. Firefox 3 beta 4 as some really impressive JavaScript performance and outruns Internet Explorer 7 with about 700%.
But the article did not test Internet Explorer 8 beta, which I think should have been there (at least for reference). Since the IE8 team claims the performance is so much better, and my feeling after a few days of IE8 usage says so I had to do some testing on my own.