"With the release of Firefox 4 Beta 7 this week, Mozilla has returned to near the top spot in browser performance rankings. According to SunSpider JavaScript benchmark suite tests run by Computerworld, the new browser is about three times faster than the current production version of Firefox in rendering JavaScript, and lags behind only Opera among the top five browser makers. Mozilla launched Firefox 4 Beta 7, a preview that includes all the features slated to make it into the final, polished version next year, on Wednesday. Beta 7 was the first to include Mozilla's new JavaScript JIT (Just In Time) compiler, dubbed 'JagerMonkey,' which shot the browser's performance into the No. 2 slot behind the alpha of Opera 11."

http://www.computerworld.com/s/article/9196258/Firefox_4_regains_speed_mojo