According to WebSideStory, which has been tracking the Firefox vs. IE numbers since June 2004, the Mozilla-made open-source browser had captured 5 percent of the U.S. market by Jan. 14, 2005, an increase of almost a full percentage point since early December. WebSideStory's "Other" category, which includes Opera and Apple's Safari, also jumped in the past month from 1.2 percent to 2.1 percent.
Internet Explorer, meanwhile, dipped from 91.8 percent in December 2004, to 90.3 percent in January. Since June of last year, Microsoft's watched IE lose slightly more than 5 percentage points of share.
WebSideStory also said that the U.S. adoption rate of Firefox had nearly tripled during the past month, indicating that the six-month assault on IE is in no danger of dissipating.
As of Thursday, Firefox's developers claimed that more than 19 million copies of the browser have been downloaded since its release 10 weeks ago.