The open source Firefox browser continues to eat into Microsoft Internet Explorer's market share, which has now fallen below 90%.
Web monitoring company Net Applications said Firefox, developed by the Mozilla Foundation, took 6.17% of the market in February compared with 5.59% in January.
Mozilla said Firefox had been downloaded 27 million times since its launch at the end of last year.
Net Applications gave Internet Explorer 89.04% of the market, compared with 90.31% last December. IE's fall below 90% had been predicted earlier this year by web monitoring company Websidestory.com.
Net Applications said that as the market share of all other major browsers had been stable over past months, Firefox's bigger slice of the total browser pie came at the expense of IE.
The Mozilla Foundation was set up a result of the spin-off of open source technology by Netscape, whose main browser used to have 80% of the market before it was usurped by IE.
Microsoft is attempting to hit back against critics, who claim IE is not secure, by releasing a beta of IE 7 this summer.
Version 7, designed for users of Windows XP Service Pack 2, will offer better protection against phishing, spyware and other malware.
Ironically, Firefox's mounting challenge to IE comes as it experiences its own niggling security vulnerabilities. Firefox has fallen victim to at least two major security holes this year.