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.