Microsoft Internet Explorer is continuing to lose share in the browser market, as its much-smaller competitors chisel at its dominant position, new Web site visitor data shows.

Microsoft's browser has dropped by 1.8 percentage points over the past three months to 93.7 percent of the market, according to data provided Wednesday by Web analytics vendor WebSideStory Inc.

The latest data comes after IE's market share fell a percentage point between June and July in the wake of a series of high-profile security issues—the first time WebSideStory had recorded an IE drop.

The benefactors of Microsoft's slight, but sustained, decline since June have included the open-source browsers from the Mozilla Foundation and a commercial competitor from Opera Software ASA.

WebSideStory's data categorized the alternative browsers into two categories, one for Netscape and Mozilla browsers and a catch-all for other browsers including Opera. Netscape/Mozilla browsers gained the most, rising 1.7 percentage points to 5.2 percent, while the category that includes Opera's browser rose 0.1 percentage points to 1.1 percent.

Mozilla itself also is reporting a record number of downloads for the preview version of Firefox 1.0, which was released Tuesday. In the first 24 hours after posting the Firefox preview, Mozilla recorded 300,000 downloads, the most in a single day for any of its applications, foundation spokesman Bart Decrem said.

That level of downloads represents 10 percent of Firefox 0.8's total downloads over its first four months of availability, Decrem said.

Microsoft, of Redmond, Wash., downplayed the latest visitor data and said in a statement that consumers, IT administrators and developers still view the IE browser as their best choice.

"We certainly encourage customers to examine all browser options, but we have not seen a significant shift in usage away from IE," a Microsoft spokesman said in a statement.

But Microsoft's competitors plan to turn up the heat. Mozilla launched a new grass-roots marketing campaign this week called Spread Firefox, and the level of traffic to the site has led to sporadic site outages, Decrem said.

The nonprofit also has set a goal of 1 million downloads of the Firefox preview within the first 10 days, along with a loftier goal of 10 million downloads within 100 days once the final Firefox 1.0 is out later this year, Decrem said.

"What we're interested in is making sure Web developers and operators of Web sites program their sites to work with standards," Decrem said. "We want to make sure that there are enough users out there that [they] are sure their site renders in Firefox."

Earlier this week, Opera also issued a news release heralding a recommendation from the German Federal Office for Information Security that users consider alternatives to IE such as Opera to avoid security issues. The U.S. Computer Emergency Readiness Team gained widespread attention in June for a similar advisory.

Microsoft isn't alone in security vulnerabilities. Mozilla itself fixed 10 security issues this week when it released Firefox and updates to the Mozilla suite and the Thunderbird e-mail client. But Decrem points out that they were fixed proactively, with many reported in the group's bounty program that rewards people for finding security vulnerabilities.