Microsoft's nightmare--well, one of them--is tucked away in the back of a Mountain View, Calif., office park. Decorated in "Mad Max" style (exposed wires, weird equipment lying around and a huge model of a suspension bridge built from soda cans on which a plastic Godzilla rages), the Mozilla Foundation employs only a dozen or so programmers. But they are the tip of an iceberg of thousands of geeks who, via the participatory open-source model, donate their time and brainpower to creating software that presents an alternative to the mighty empire in Redmond. Notably, a Web browser called Firefox.

In the last few months a growing number of Just Plain Users have been abandoning Internet Explorer, the browser Microsoft built to go with its operating system, for Firefox. Though Explorer is still by far the champion, its 90-plus percent market share is shrinking a bit every month.

For Bill Gates, the idea of a movement to some other browser is a disruption to the natural order of things. "Explorer is going to be the primary browser used on Windows, and for anyone to suggest otherwise is just irresponsible," he told me last October. Seventeen million Firefox downloads later, it's clear that rhetoric alone won't stem this tide.

Why are techno-savvy users getting Foxy? Reason one is that Microsoft, after a successful predatory assault on its main browser rival, Netscape, simply stopped innovating on its own product. Firefox is only one of several browsers (including Opera and Apple Computer's Safari) that have recently emerged with exciting new features, such as "tabbing" (to easily navigate multiple sites) and full support for the hot new RSS technology (which lets you "subscribe" to Web sites). In addition, the sleek and nimble Firefox presents a more pleasant experience in general. "People had come to think that browsers didn't matter, but if you have a better one, the whole way you browse the Web is more pleasant," says Mozilla Foundation president Mitchell Baker. And switching to Firefox is easy--a free, quick download (mozilla.org), and the program will even transfer your bookmarks, your history and even your passwords from Explorer.

But perhaps the biggest motivator is safety. Explorer users are routinely victims of security breaches and spyware invasions; Microsoft's recent security update, SP2, has helped but not solved the problem. Because Explorer is so tightly integrated with Windows, says Firefox's chief architect Brendan Eich, vulnerabilities give outsiders access to your entire computer. "We get mail all the time saying, 'Thank you for Firefox--I was about to throw out my computer'," he says. (Microsoft Windows head Will Poole responds: "The specific vulnerabilities that have been exploited in Explorer in the past may not exist in other browsers, but other browsers have their vulnerabilities, as well." Also, Poole says that when it comes to making improvements to Explorer, Microsoft is constrained by its unique responsibility to make sure it works smoothly with all existing Web pages.)

How many defectors can Firefox ultimately lure? Depends. Eyebrows were raised when the Mozilla Foundation bought a two-page spread in The New York Times last month, a sign of high aspirations. But to move well into double digits of market share, the Firefoxers will have to persuade computer makers and ISPs to include the program in their initial installations.

Nonetheless, Firefox's success proves that the open-source process can reach beyond geekdom to serve--and even delight--Just Plain Users. And that when the Empire has no clothes, people will go out and find some new threads on their own.