"Firefox 9 is now available � but unlike its previous rapid release forebears where not a lot changed, a huge feature has landed with the new version: the JavaScript engine now has type inference enabled. This simple switch has resulted in a 20-30% JS execution speed increase (PDF), putting JaegerMonkey back in line with Chrome's V8 engine, and even pulling ahead in some cases. If you switched away from Firefox to IE or Chrome for improved JS performance, now is probably the time to give Firefox another shot."