(FORTUNE Magazine) – "DITCH YOUR BROWSER," WRITES EDITOR-IN- chief Harry McCracken in October's PC World, a magazine widely read by techies and power users. What on earth is he talking about? Like a growing number of techies, McCracken and many of his staff have converted to Firefox, browser software designed to replace Microsoft's popular, hugely successful Internet Explorer. The upstart program (free at www.mozilla.org) now accounts for some 3% of the U.S. desktop market, according to web-analytics firm WebSideStory. That number sounds puny until you consider that it means roughly four million PCs. And while Internet Explorer retains a still overwhelming market share of 93%--it is used on about 115 million laptops and desktops--Firefox is spreading like, well, wildfire: It snatched all three of its points of market share between June and October.

Firefox is hot mostly for one sad reason: Crooks, hackers, and ethically challenged online marketers are using security weaknesses in Internet Explorer to attack people's PCs and privacy. They write viruses that hobble Explorer, as well as "spyware" that sneaks onboard our machines to track the websites we visit and steal our personal data--sometimes even our credit card numbers.

Microsoft's browser is the target precisely because it is ubiquitous. Successful exploits against it can wreak vast havoc. A recent study by America Online found that 80% of consumer PCs are afflicted with spyware, with the average machine hosting an amazing 93 pieces of invasive software code. Until recently Microsoft compounded the problem by being slow to plug IE's security holes. This year cyber-security agencies of both the U.S. and German governments suggested users consider switching to alternative browsers to avoid IE's troubles.

Firefox is the biggest beneficiary of IE's woes, along with other browsers like Opera and Apple's hot Safari for the Macintosh, which many consider the state of the art. At least so far, spyware and viruses do not much affect Firefox--miscreants prefer the big, juicy target of IE. So while Firefox and Explorer are roughly equivalent in raw performance speed, Firefox is often faster at calling up pages. That's because a spyware-infected IE browser wastes lots of computing time satisfying the demands of the parasitic software. For every web page you navigate to, spyware may be compiling data and sending reports elsewhere, or calling up a pop-up ad.

Many say Firefox is also more fun to use; it dazzles people with its user-friendly design. For instance, it includes so-called tabbed browsing to keep open multiple windows with different websites. You can even open your favorite ten sites each morning with a single mouse click. Want to block an ad? A single click does it. Search services from Google, Yahoo, Amazon, eBay, and Dictionary.com are built in.

Does all this mean you should switch to Firefox? Not necessarily. But--here's the important part--the problems with Internet browsers almost certainly apply to you. If you're like most plugged-in people today, you increasingly lean on your browser as an essential tool. And with spyware and viruses on the rise, you need to take action, just as you would if the lock on your house were broken.

Microsoft gained dominance in browsers in the late 1990s, after Netscape pioneered the market with its Navigator software. As millions of people discovered the Internet, Bill Gates belatedly realized that the browser could become as important to users as Windows desktop software itself. Determined to seize Netscape's franchise, Microsoft built Internet Explorer and started giving it away for free.

In a sense, Firefox is the ghost of Netscape, rising up to vex its conqueror. In 1998 the battered startup donated Navigator's underlying code to a public-domain "open source" effort called the Mozilla project. (Netscape is now owned by Time Warner, parent of FORTUNE's publisher.) At the time, few took much notice. But six years later the nonprofit group now called the Mozilla Foundation has, with the help of an army of mostly volunteer programmers, refined the code into a full-blown Explorer rival. Says consultant and longtime web expert Jerry Michalski: "The Mozilla team has turned Firefox from a huge tangle of spaghetti code into an elegant, innovative browser."

The project is winning support--in labor and funds--from Microsoft's commercial rivals, including IBM, Red Hat, Nokia, Novell, and Sun. Foundation president Mitchell Baker, a former Netscape lawyer, is confident that her product will continue to win converts. "People are unhappy with their web experience," she says, "and we have a product that improves that experience dramatically." Some websites aimed at techies report that 20% of visitors or even more now use Firefox.

For its part, Microsoft has vastly intensified its efforts to provide software security. If you want to avoid IE problems, the company says, never mind Firefox. Instead, you should install the latest update to Windows, released in August and known as Service Pack 2. It includes a raft of security, safety, and antispyware features for Windows and Explorer. SP2 is free, but it can take hours to install. What's more, it only works with Windows XP; if you have an earlier version of Windows, upgrading to XP costs at least $80. But Microsoft product manager Gary Schare argues that using a computer is much like driving a car--periodic maintenance is just part of the deal. "If you never change the oil, you're going to have a problem," he says. "We urge people to get SP2 to be as safe as possible."

Many tech-savvy users argue that switching to Firefox is simpler. Yet that too has its downsides. Most websites optimize their content for IE; if you arrive with another browser, things might not look right. And no matter which browser you opt for, every Windows user occasionally needs IE. For example, you have to use it to arrange automatic Microsoft security patches on its website. You also need IE to employ some of the hottest web innovations, like the Google search toolbar. Eric Peterson, an analyst at Jupiter Research, thinks it makes more sense to avoid the hassle of switching and stick with IE. (Vigilance and regular updating remain essential no matter which browser you use.)

Microsoft had better hope the Firefox uprising is not portentous, says Greg DeMichillie, an analyst at Directions on Microsoft, a Kirkland, Wash., research firm. The software giant has never before faced significant market erosion involving a product with which it had achieved a monopoly, he explains. "Firefox in itself doesn't represent a significant business threat," he says, "but if it shows that open-source products can take market share, then maybe the same thing could happen in Office [Microsoft's suite of desktop applications]. Now we're talking real money, because Office is where Microsoft gets half of its revenues." DeMichillie, by the way, is a Firefox user.

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