According to this article from CNet News, Firefox is drawing fans away from Microsoft IE. The most significant numbers are:
"The percentage of visitors to e-commerce and corporate sites that used Firefox or another Mozilla browser grew to 5.2 percent in September, from 3.5 percent in June 2004. Meanwhile, Microsoft's share of the users shrank from 95.5 percent in June to 93.7 percent in September, according to the company [WebSideStory]."
"W3Schools.com, a Web development tutorial site found that Mozilla-based browsers were used by 18 percent of its visitors in September, up from 8 percent in January. Internet Explorer use dropped to 75 percent from 84 percent in the same period.
Among CNET News.com readers, site visitors with the Firefox and Mozilla browsers jumped to 18 percent for the first two weeks of September, up from 8 percent in January."
Actually, I like the W3Schools.com statistics about the browser market share. But it lacks of market, sites and web apps segmentations.