Internet Explorer last month saw its market share drop for the first time this century, according to WebSideStory. Total market share fell by 1 per cent in June.

"It's the first time that we've seen a sustained trend downward for (Microsoft)" said Geoff Johnston, an analyst with WebSideStory. "We have a very steady trend. It's been about a month, and every day we have a steady incremental change."

Internet Explorer has held more than 95 per cent of the browser market since June 2002, and until June had remained steady with about 95.7 per cent of the browser market, according to WebSideStory's measurements. Over the last month, however, its market share has slowly dropped from 95.73 per cent on June 4 to 94.73 per cent on July 6.

A loss of 1 per cent of the market may not mean much to Microsoft, but it translates into a large growth, proportionately, in the number of users running Mozilla and Netscape-based browsers. Mozilla and Netscape's combined market share has increased by 26 per cent, rising from 3.21 per cent of the market in June to 4.05 per cent in July, Johnston said.

"It takes a lot to get someone to change their browser. It's been years since anyone has been willing to do this in significant numbers," he said.

WebSideStory's estimates are based on a daily survey of about 30 million browsers hitting thousands of different Web sites that use the company's Web analytics software, Johnston said.

Downloads of the Mozilla Foundation's Firefox browser have been increasing since version 0.8 of Firefox was released in February, said Mozilla spokesman Bart Decrem, but the open source project saw a major spike in downloads at the end of June, following reports of the so-called Download.Ject vulnerability in Internet Explorer that could allow attackers to trick users into loading insecure content.

A June United States Computer Emergency Readiness Team (CERT) advisory recommending users stop using IE contributed to Mozilla's momentum, as did a number of favourable reviews in mainstream publications such as USA Today and Slate, Decrem said.

Because Mozilla browsers do not use the "Trusted Zones" security model employed by Microsoft, they are less vulnerable to attacks like Download.Ject, he said.

Of course, Mozilla isn't immune to security flaws. Earlier this week, Mozilla developers issued a patch for a browser vulnerability that could allow an attacker to execute existing applications on a Windows system.