Firefox 3 (3.1) vs. Google Chrome vs. Safari 3 vs. Opera 9.5 vs. Internet Explorer 8 (IE8) vs. Internet Explorer 7 (IE7)

John Resig did a great job by comparing Firefox 3, Firefox 3.1, Google Chrome, Safari 3, Opera 9.5, Internet Explorer 8 and 7 JavaScript performance.

JavaScriptCore: The engine that powers Safari/WebKit (up until Safari 3.1).
SquirrelFish: The engine used by Safari 4.0.
V8: The engine used by Google Chrome.
SpiderMonkey: The engine that powers Firefox (up to, and including, Firefox 3.0). Read more