The new JavaScript interpreter in Firefox 3.1 uses an optimization strategy called tracing to achieve 20 - 40 times faster performance. This is really pretty incredible since, in my experience, Firefox 3.0 is already pretty speedy when executing JavaScript code.

Mozilla has adopted an innovative new optimization technique for the Firefox JavaScript interpreter. Benchmarks show that JavaScript execution is now up to 40 times faster in some benchmarks. Mozilla aims to deliver performance comparable to that of native code.

Firefox to get massive JavaScript performance boost

This should allow developers to do much more complex tasks in JavaScript and still achieve acceptable speed. Now if someone would come out with a speedy JavaScript plug-in for Internet Explorer 6 & Internet Explorer 7 developers could expect the same performace from all widely used browsers.