In ajaxian.com there is a benchmark for the javascript engines of several builds of browsers. Very impressive results..

The results of the SunSpider JavaScript Benchmark test

1. Firefox 3 Nightly (PGO Optimized): 7263.8ms
2. Firefox 3 Nightly (02/25/2008 build): 8219.4ms
3. Opera 9.5.9807 Beta: 10824.0ms
4. Firefox 3 Beta 3: 16080.6ms
5. Safari 3.0.4 Beta: 18012.6ms
6. Firefox 2.0.0.12: 29376.4ms
7. Internet Explorer 7: 72375.0ms