Hailed only months ago as the long-awaited savior to discontented users of Microsoft's Internet Explorer (IE), the Mozilla Foundation's Firefox open-source browser now is being subjected to the kind of scrutiny and criticism that inevitably confronts any challenge to the status quo.

If you haven't heard the latest, it turns out Firefox isn't the impenetrable fortress of browsing security some may have assumed it to be. Last week, a security company issued a warning about flaws found in Firefox 1.0.3 that make it vulnerable to existing exploit code. (Mozilla released a security update fixing the problems a couple of days later.) And this week, Executive Tech columnist Brian Livingston reported on some other security flaws in Firefox.

Let me say right up front that I use Firefox and prefer it greatly to IE. I switched late last year at the urging of the computer-repair pro who recommended it while billing me $180 to cleanse my hard drive, which had taken on a new identity as the Museum of Modern Malware. If it was in the wild, it was on my machine... courtesy of IE.

Still, I've never had any illusions about perfection in the technology world, and that includes Firefox. For example, I get some pop-ups using Firefox, even though I have a check-mark right next to ''Block Popup Window'' under the Options feature. And Firefox has crashed on me.

The fact is, Firefox isn't impervious to malicious code, and it's not perfectly safe. But ''not perfectly safe'' hardly is another way of saying ''just as bad as IE''. And as someone who believes genuine competition is better for technology users, I'm worried that IE users who might have been tempted to switch to Firefox may back off because they think the open-source browser is no better than Microsoft's enabler of malware mayhem.

Indeed, the latest figures from Web site measurement and marketing firm WebSideStory, released last week, show a second consecutive slowdown in monthly market-share gain for Firefox. Through April 29, Firefox was at 6.8 percent browser usage in the U.S., while IE had dipped to 88.9 percent.

Prior to the release of Firefox 1.0 last November, the notion of IE falling below 90 percent market share was unthinkable. Last June, IE owned nearly 96 percent of the U.S. browser market. Just weeks after Firefox's debut, though, that number was down to 92 percent. By Feb. 18, IE had dropped to 89.9 percent, while Firefox jumped to 5.7 percent from 4 percent.

If Firefox's growth rate continues to slow, it will be a real challenge for Mozilla to reach its goal of 10 percent market share by year's end. And just as any public company can be punished on Wall Street for missing financial targets, it's possible that a flurry of ''Firefox Misses Market Share Goal'' headlines could create a deadly psychological barrier in the browser market.

And that would be a shame, because Firefox has a long way to go to become a legitimate alternative to IE, especially in the enterprise. One analyst quoted in this recent Datamation story says Firefox won't be viable in the enterprise ''until it reaches at least a 20 percent share.''

I'm not madly in love with Firefox. I'll dump it faster than 4,000 shares of ImClone if something better comes along. That's what I -- and millions of others -- did to AltaVista when Google arrived on the scene.

The truth is, when it comes to Internet tools, brand loyalty is a tenuous notion at best, especially if there is no cost of conversion. (Which, of course, is where Redmond usually comes in.)

I hope users are quick to point out flaws in Firefox, just as they are for IE. I hope Mozilla continues to respond quickly, as Microsoft often (too often) has. But mostly I hope Firefox continues to gain market share so we can have a real choice.