Mozilla's new Firefox 3.6 is about 15% faster than its predecessor, Firefox 3.5, but still is a slowpoke compared to the current speed demon s, Apple's Safari and Google's Chrome, benchmark tests show.

According to tests run by Computerworld , Firefox 3.6, which Mozilla launched on Thursday, is the third fastest of five Windows browsers tested. Firefox renders JavaScript three times faster than Opera 10 and more than four times faster than Microsoft's Internet Explorer 8 (IE8). It's also 14.5% faster than Firefox 3.5, the Mozilla browser that debuted in June 2009, a slightly larger speed increase than Mozilla has claimed.

But even with the JavaScript speed boost, Firefox 3.6 can't match Safari or Chrome. Safari is twice as fast -- and Chrome 4.0 nearly twice as fast -- as Firefox.

Safari edged Chrome for first place in the speed race, beating Google's browser by about 6.5%, a slightly smaller lead than in a November time trial that pitted Mac versions of the browsers against each other.

Computerworld ran the SunSpider JavaScript benchmark suite in Windows XP Service Pack 3 (SP3) three times for each browser, then averaged the scores to arrive at the final rankings.

Most browser makers have been aggressively promoting improved JavaScript performance for nearly two years now, dating to when Mozilla began touting the performance boost its new TraceMonkey JavaScript engine gave to what eventually was tagged as Firefox 3.5 and Google's trumpeting the raw speed of Chrome.

Even Microsoft, which had disparaged speed tests, has acknowledged its developers are working on giving the next version of IE a kick in the pants. In November, Steven Sinofsky, Microsoft's president of Windows and Windows Live, claimed that early work on IE9 put the browser's JavaScript speed "on par" with rivals. Microsoft has not yet unveiled a test or preview build of IE9 that can be independently tested, however.

David Mandelin, who works on Mozilla's JavaScript team, has posted technical details on Firefox 3.6's JavaScript improvements on the hacks.mozilla.org blog.

Mozilla made other changes to Firefox 3.6 to speed up the browser. The Mac edition starts up about 30% faster than Firefox 3.5, for example, while changes to the location bar -- the searchable address bar that Mozilla dubs the "Awesomebar" -- on all versions have resulted in what the company claimed were "massive improvements in [user interface] responsiveness when typing in the location bar."

On another oft-quoted scoring system, the final of Firefox 3.6 did not budge from earlier beta builds of the browser, however. As it had last year during its beta testing, the production version of Firefox 3.6 scored 92 out of a possible 100 on the Acid3 benchmark , which checks how closely a browser follows standards related to DOM (Document Object Model), CSS2 (Cascading Style Sheets) and SVG (Scalable Vector Graphics).

Current editions of Safari, Chrome and Opera all score 100 on the Acid3 test, while IE8 reaches only 24.

Firefox currently accounts for about 25% of all browsers used worldwide, according to the most recent data from Web metrics company NetApplications.com. With a quarter of the browser market, Firefox is a distant second to IE's 63%, but enjoys a huge cushion over the current No. 3 browser, Chrome, which has a 5% share.

For more on Firefox 3.6, check out Computerworld 's review here.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld . Follow Gregg on Twitter at @gkeizer, send e-mail to gkeizer@ix.netcom.com or subscribe to Gregg's RSS feed.

This story, "Firefox Takes a 15% Speed Jump" was originally published by Computerworld .