Mozilla may have switched on a new JavaScript rendering engine in the just-released Firefox 3.1 Beta 2, but the browser remains slower than preview versions of two rivals, benchmark scores indicate.

According to tests run by Computerworld, Firefox 3.1 Beta 2 trails both WebKit, the open-source project that provides the core engine for both Google's Chrome and Apple's Safari, and the most recent version of Chrome itself, in rendering JavaScript. Mozilla's beta, however, is significantly faster than the current production edition of Firefox, and beats the preliminary version of Microsoft's Internet Explorer 8 by a wide margin.

Computerworld ran the SunSpider JavaScript benchmark suite in Windows XP three times for each browser, then averaged the scores. In SunSpider, smaller numbers are better. The results were:

1. WebKit, (r39076) -- 900 2. Chrome 0.4.154.33 -- 1265 3. Firefox 3.1 Beta 2 -- 1775 4. Opera 10 alpha -- 3419 5. Firefox 3.0.4 -- 3454 6. Safari 3.2 -- 3574 7. Opera 9.6.2 -- 4007 8. IE8 Beta 2 -- 9584

The results show that the December 7 build of WebKit is nearly twice as fast as Firefox 3.1 Beta 2, while the newest developer version of Chrome is about 40 percent faster. Both WebKit and Chrome -- the latter relies on the former for its foundation -- use the "SquirrelFish Extreme" JavaScript engine, a relatively recent upgrade that WebKit developers first unveiled in September. Firefox 3.1, on the other hand, includes Mozilla's new TraceMonkey JavaScript interpreter. Beta 2 is the first public preview to switch on TraceMonkey by default.

But while Firefox 3.1 lags behind WebKit and Chrome, it is substantially faster than every other browser that was tested. Beta 2, for example, is almost twice as fast at rendering JavaScript as Opera Software's Opera 10 , which was released in alpha just last week; and Mozilla's current production Firefox 3.0.4 version. It's also more than twice as fast as Opera 9.6.2 and Apple's Safari 3.2.1, and more than five times faster than Internet Explorer 8 Beta 2.

Mozilla, which began work on TraceMonkey this year, announced the new engine in August. At the time, Mozilla said that Web 2.0 applications required faster JavaScript. Just weeks later, shortly after Google launched Chrome, Mozilla decided to delay Firefox 3.1 so it could add, among other things, the TraceMonkey engine.

Firefox 3.1, which is slated for an early 2009 release, will go through at least one more beta, Mozilla said two weeks ago, so that programmers can evaluate TraceMonkey and other new features.