At the end of September, IE's share dropped to 49.87%, StatCounter reported Tuesday. Just two years ago, IE dominated the market with a 67% share.
"This is certainly a milestone in the Internet browser wars," Aodhan Cullen, chief executive of StatCounter, said in a statement.
The research firm found that the rising star in the browser market was Google's Chrome, which has tripled its share to 11.54% in September from 3.69% the same month a year ago. In June, Chrome overtook Apple Safari for the first time in the U.S. Mozilla Firefox holds the second largest market share with 31.5%.
Microsoft has been the powerhouse in the browser market since the company started bundling IE with Windows in order to compete with Navigator, a browser launched in 1994 by Netscape Communications Corp. By leveraging its Windows monopoly, Microsoft contributed to the eventual demise of Netscape.
Today, government regulators require Microsoft to create a level playing field for competing browsers in Windows. In Europe, European Commission competition authorities require Microsoft to provide a menu of browsers to give Windows users a choice.
The stiff requirements in Europe have contributed to IE's share falling to 40.26% in September from 46.44% a year ago, StatCounter said. In North America, IE still accounts for more than half of the market at 52.3%, followed by Firefox at 27.21% and Chrome at 9.87%.
StatCounter bases its report on data collected from more than 15 billion page views per month at the more than 3 million websites in the StatCounter network. The firm provides a Web site tracking and reporting service.
Net Applications, another Web metrics firm, has drawn other conclusions on the global browser market. The company this month reported that IE's share had fallen in September but still accounted for more than half the market at 59.65%.
Findings from research firms often differ due to differences in methodology.