Firefox 3.0 is dramatically faster than its predecessor and rivals, benchmark tests claim -- the result of hundreds of performance improvements designed to make the open-source browser the best at running complex Web 2.0 applications, Mozilla's chief developer said Wednesday.

"We've been working on performance for a long time," said Mike Schroepfer, Mozilla's vice president of engineering. "Each beta of Firefox 3.0 got better. Beta 1 was better than Firefox 2.0, Beta 2 was better than Beta 1 and so on. Some of the big architectural changes [we've made] had begun paying off. Now we're at the point where we can turn the knob to get it to perform well."

Firefox 3.0 Beta 4, which Mozilla released late Monday, has been put through its paces by users and bloggers, some of whom have published the results of head-to-head benchmark tests between Firefox, Opera, Apple Inc.'s Safari and Microsoft Corp.'s Internet Explorer. According to Percy Cabello, who posted his results on the Mozilla Links blog, Firefox 3.0 Beta 4 is 53% faster than Opera 9.5 beta, twice as fast as Apple's Safari and three times faster than Microsoft's IE7 on the SunSpider benchmark, which tests JavaScript performance.

Schroepfer, however, refused to be drawn into a conversation about benchmarks. Instead, he talked about what the open-source project is looking to do.

"There are lots of ways to 'game' the system [in benchmarks], but what we're trying to do is speed up the things that enable people to run the really heavy-duty applications on the Web."

Saying that a browser is a browser is a browser when rendering basic Web pages, Schroepfer added that the challenge Mozilla took on was how to build a browser that performs well when asked to run much more complex Web 2.0-esque applications, such as Yahoo's Zimbra, the online collaboration and document suite.

"Web apps today are magnitudes more complex than those from five years ago," said Schroepfer. "When Yahoo started out, it wasn't anything more than a bullet list. Now it has widgets and word processing. It's important for us to make it possible for Web designers to create complex applications. They can be confident building [big Web applications] knowing that Firefox can handle them."

Firefox's developers have dealt with more than 400 performance-related bugs and changes, Schroepfer said. "There are a bunch of things that have to come together to get these kind of results," he said, as he ticked off several. "We optimized JPG encoding, developers took advantage of more and newer compiler options and we found a way on Mac OS X to keep it from throttling page rendering."

Larger-scale modifications included new graphics- and text-rendering architectures in Gecko, Firefox's engine, and a completely revamped JavaScript engine.

Boosting Firefox's performance is also important, Schroepfer said, for the mobile market, which Mozilla has begun exploring. "The performance gains carry over into mobile, where underpowered devices are the rule," he said. "It should really help us there."

Mozilla is at the end of the line in the mobile space. There, Opera Software ASA's Opera leads all others in popularity on handsets. IE, found on Windows Mobile-powered smart phones, and Safari on the iPhone trail Opera in installations, but are ahead of Firefox by miles.

"I think better performance goes back to the mission of Mozilla," concluded Schroepfer. "That mission is to move the Web as a platform forward." That's happening, he argued, with Firefox leading the way.

"I'm happy to see that other browsers are working on performance now, too."

Firefox 3.0 Beta 4 can be downloaded for Windows, Mac OS X and Linux in 36 languages from Mozilla's site.