As Firefox marks its 5 millionth download and moves to retail stores, a survey showed Microsoft’s heavyweight Internet Explorer slipping below 90 percent market share.

OneStat.com, an Amsterdam, Netherlands-based provider of Web traffic measurement services, said that according to numbers gleaned from 2 million Web surfers around the world, the Mozilla Foundation’s new Firefox releases appear to be cutting into Microsoft’s still commanding lead in the browser market.

„It seems that people are switching from Microsoft’s Internet Explorer to Mozilla’s new Firefox browser,” Niels Brinkman, OneStat.com co-founder, said in a statement.

OneStat’s statistics, based on the Web surfers’ activity in 100 countries, showed that IE’s market share slipped to 88.9 percent in the third week of November, down 5 percentage points from its share in May.

Mozilla-based browsers, including Firefox, rose to 7.4 percent, up 5 percentage points from May.

The new figures come on the heels of another survey that showed IE on a downward trend. Earlier this month, WebSideStory reported that IE was continuing to slip, claiming 92.9 percent of Web surfers in October versus 95.5 percent in June.

Mozilla heralded its download numbers as a sign of progress for the open-source browser.

„We’re seeing a much swifter uptake for 1.0 than for the preview release, which took more than a month to reach 5 million downloads,” Asa Dotzler, Mozilla’s release manager, said in a statement. „We’re clearly reaching a new world of users and we’re doing it at a faster pace than any time in Mozilla’s history.”

In addition to the increase in downloads, Firefox can look forward to retail sales in the form of a new offering by Linspire, the open-source software distributor, which said Monday it had unveiled a product called „OOoFf” that combines the open-source productivity software suite OpenOffice.org with Firefox.

The software package, available now from the OOoFf Web site, is available for use with the Microsoft Windows, Apple Macintosh and Linux operating systems. It costs $29.95 when ordered online, with free shipping. It also will be sold at retail stores, according to Linspire.

Firefox advances aside, Microsoft threw cold water on the OneStat.com numbers, saying they failed to account for IE’s stronghold in the corporate world.

„It doesn’t jibe with what WebSideStory shows, and what neither of these count is corporate intranets where users aren’t actually hitting the Web,” said Gary Schare, Microsoft’s director of product management for Windows.

Schare attributed Mozilla’s spike in popularity to interest by „early adopters,” and predicted that in the end IE would retain its majority status.

„I still believe in the end that most users will decide that IE is the best choice when they take into account all the factors that led them to choose IE in the first place,” Schare said. „Meanwhile, we’re happy that they’re primarily (using Firefox) on Windows, and that Firefox is part of the large ecosystem of software products available on the Windows platform.”