Mozilla's Firefox 4's browser usage share grew over two-and-a-half times in the six days since its March 22 launch, a Web analytics company said today.
According to California-based Net Applications, Firefox 4 accounted for 3.7% of all browsers used Sunday, up from last Tuesday's 1.4%.
Firefox 4's Sunday share was double that of Microsoft's newest browser, Internet Explorer 9 (IE9), which debuted March 14.
Mozilla has been touting Firefox 4's download count since the browser launched last week, when it recorded 7.1 million downloads in the first 24 hours, and an unofficial record of 8.75 million the second day.
Those downloads have translated into a growing share for Firefox 4, which has averaged a 2.8% usage share since its release six days ago.
By comparison, IE9 is off to a much slower start, climbing just six-tenths of a percentage point -- an 80% increase -- and averaging 1.1% during its first six days. Since its launch two weeks ago, however, IE9 has doubled its usage share.
Firefox 4's battering of IE9 in the usage share battle wasn't unexpected. Unlike IE9, Mozilla's browser runs on Windows XP, Mac and Linux, operating systems that Microsoft's program doesn't support.
IE9 runs only on Windows Vista and Windows 7, which collectively account for slightly more than a third of all versions of Windows now in use.
Microsoft and Mozilla have traded barbs over the former's decision to dump XP. Microsoft has defended the move by saying it did not want to develop for what it called "the lowest common denominator," while Mozilla has bashed its rival for leaving XP users stuck with the two-year-old IE8.
It's unclear whether the releases of Firefox 4 and IE9 have managed to boost the overall usage shares of Mozilla or Microsoft, or if the gains of the new editions have come at the expense of older versions of the firms' browsers. Net Applications will not release its March numbers until Friday.
Both Firefox and Internet Explorer have been on a downward trend for months. Firefox has lost 2.5 percentage points in the last 12 months, while IE has dropped 4.8 points in the same period.
Google's Chrome and Apple's Safari have been the beneficiaries of those declines, picking up 5.3 and 1.9 points, respectively, in the last year.
Net Applications calculates browser usage share using data obtained from the 160 million unique visitors who browse the 40,000 Web sites the company monitors for its clients.