According to California-based Net Applications, one of a handful of companies that regularly publishes browser usage data, IE lost eight-tenths of a percentage point of share in April, falling to 55.1 percent, a new low for Microsoft.
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Meanwhile, Firefox dropped two-tenths of a percentage point to 21.6 percent, a share equivalent to its December 2008 standing.
Both Microsoft and Mozilla debuted new browsers several weeks ago: The former launched IE9 on March 14, while the latter shipped Firefox 4 on March 22. Neither release stemmed their maker's long-standing slide.
Apple's Safari and Google's Chrome took up the slack in April.
Safari posted an increase of five-tenths of a point to end the month at 7.2 percent, a record for the browser that ships with Mac OS X, and is integrated with iOS, the mobile operating system that powers the iPhone and iPad. The one-month gain was Safari's biggest-ever by Net Applications' tracking.
Chrome's usage share grew by four-tenths of a percentage point, slightly less than the browser's average increase over the past 12 months. Google's browser accounted for 11.9 percent of all browsers used in April.
Opera, the Norwegian browser that rounds out the top five, was flat last month at 2.1 percent.
Data from Irish analytics company StatCounter differed from Net Applications' in share amounts for each browser, but showed the same trends: IE and Firefox losing share, Safari and Chrome gaining.
But while Net Applications had IE and Firefox down overall, the newest versions attracted existing users.
IE9 accounted for 2.4 percent of all browsers, a 1.4-point increase, nearly three times March's gain. The surge was likely fueled by the IE9 upgrade offer that Windows 7 and Vista users started see via Windows Update on April 18.
But highlighting Microsoft's difficulty recouping lost share, IE9's growth was almost exactly offset by a 1.4-point drop in the older IE8, the browser that shipped with Windows 7 and which most Vista users have been running since mid-2009.
All of Microsoft's older browsers continued their steady fall. IE6, the browser Microsoft has campaigned to kill, fell by one-tenth of a point to 10.9 percent, while IE7 slipped by five-tenths of a percentage point to 7.4 percent. And IE8 dropped for the second consecutive month to end April at 33.1 percent, a number about equal to its July 2010 share.
As it has done before , Microsoft again ignored IE's overall decline to focus instead on talking up IE9's progress on Windows 7. On that edition of Windows, the new browser averaged a 7.5 percent share for April -- double the month before -- and neared a 10 percent share by the end of April.
"Just six weeks after the launch of Internet Explorer 9 we are about to reach a new milestone, as of the last day in April, IE9 is at 9.95 percent usage share worldwide on Windows 7," said Ryan Gavin, senior director of IE, in a Sunday blog post.
Gavin also reiterated Microsoft's assertion that IE9 was the best browser for Windows 7, an argument based on the company's belief that other browser makers " dilute their engineering investments" because they support older versions of Windows as well as Mac OS X and Linux.
But users clearly disagree with Microsoft, at least for now.
According to Net Applications, Chrome 10, Firefox 3.6, and Firefox 4 all had larger shares on Windows 7 last month. Chrome 10, which Google began pushing to current Chrome users via the browser's silent update mechanism in early March, accounted for 14.8 percent of Windows 7 browsers in April. Mozilla's Firefox 3.6 owned a 14.1 percent share on Windows 7, and even Firefox 4 outpaced IE9 with 8.4 percent.
Nor did IE9's full month of availability in April improve Microsoft's overall share on Windows 7. IE's total share on the new OS stayed flat last month, while Firefox's grew by one-tenth of a point and Chrome's jumped six-tenths of a point.
Net Applications calculates browser usage share with data obtained from the 160 million unique visitors who browse the 40,000 Web sites the company monitors for its clients. Its April browser statistics can be found on the Net Applications site.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer or subscribe to Gregg's RSS feed. His email address is email@example.com.
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This story, "IE9 can't stop Microsoft's browser slump" was originally published by Computerworld.