Internet Explorer (IE) gained browser usage share last month in the U.S., while major rivals Firefox and Chrome both lost ground, Microsoft said today, citing data from Web analytics firm Net Applications.

"This is an incredibly competitive space now, which is incredibly healthy," said Ryan Gavin, director of platform strategies for Microsoft. "But we're already seeing Chrome in retreat in the U.S."

According to Net Applications data not available to the general public, all versions of IE gained 0.76 of a percentage point in U.S. usage share last month, accounting for 63.27% of the browsers used in May. Firefox and Chrome, meanwhile, fell 0.24 and 0.45 of a percentage point, respectively, in the U.S. last month, ending with shares of 20.38% and 4.53%.

Net Applications confirmed that the data Gavin cited was accurate.

But the growth of IE in the U.S. was not enough to offset its decline globally, where Microsoft gave up 0.26 of a percentage point to fall to a new low of 59.7%. Meanwhile, Google 's Chrome and Opera Software's Norwegian-made Opera boosted their worldwide shares in May at the expense of IE and Mozilla's Firefox.

By May's end, Chrome accounted for 7.05% of the browsers that surfed to the 40,000 sites that Aliso Viejo, Calif.-based Net Applications monitors for clients. Opera increased its share by 0.13 of a percentage point, its largest increase in eight months, to 2.4%.

Chrome's increase of 0.3 of a percentage point was the browser's smallest gain since August 2009, and significantly off its three- and 12-month averages of nearly half a point.

Firefox, on the other hand, was again down last month, sliding 0.24 of a percentage point to 24.35% worldwide, marking the fourth time in the last six months that the browser's share slipped. In March and April, Firefox gained back some of the ground it had lost since November 2009, but May's decline cost Mozilla most of the growth it had fought for during the two-month stretch. Firefox now stands at about the same share it had in January.

Once considered a lock to hit and then move beyond the 25% bar, Firefox has yet to reach that milestone. In April, Vince Vizzaccaro, a Net Applications executive vice president, said that Firefox was "just holding steady" and explained that gains that had once come its way were instead being gobbled by Google's Chrome.

Microsoft disputed that Chrome is grabbing share.

"IE8 continues to lead in user choice," Gavin argued. "It grew 2.5 times faster than any competitor." He arrived at the two-and-a-half times figure by comparing IE8's global increase of 0.81 of a percentage point with Chrome's growth of 0.32 of a percentage point.

IE8 closed May with 35.38% of the U.S. browser usage share, making it the most-used browser in the country. Microsoft's IE7 was second, with 16.75%, said Vizzaccaro, while Mozilla's Firefox 3.6 was third with 13%. Google's best showing was at No. 8, where Chrome 4.1 accounted for 3.52%. Chrome 5.0 , which just shifted out of beta into what Google dubs its "stable channel," owned 0.76% of the usage market.

"Microsoft is doing extremely well with IE8 in the U.S.," said Vizzaccaro in an e-mail today.

" Windows 7 [growth] is certainly part of the reason why IE8 is growing," said Gavin, when asked for Microsoft's explanation of IE's increase in the U.S. "But there is more choice in [the browser] space than at any other time in history," he said, adding that when IE8 goes head-to-head with rivals, such as in Europe, where Windows users have been offered a government-mandated way to change browsers, IE8 does well.

"IE8 grew half a percentage point in Europe last month," said Gavin, to 29.69%.

Besides touting the climb of IE8, Gavin also plugged the success that Microsoft has had in driving down the share of the nearly-nine-year-old IE6, the aged browser that Microsoft has been aggressively urging customers to ditch. He confirmed that one of his tasks is to push IE6's share to zero.

"In the U.S., we're seeing that tip-over [towards zero]," Gavin said, and again credited Windows 7 and IE8 adoption as reasons. Windows 7, the operating system Microsoft launched last October, was the first to include IE8.

Microsoft has made more headway in its "kill IE6" movement in the U.S. than it has globally. According to Net Applications, IE6 accounted for 6.74% of all browsers used last month in the U.S., a number substantially lower than IE6's worldwide average of 17.13%.

The two biggest groups of customers still running IE6, said Gavin, are enterprises having trouble migrating because of mission-critical Web applications or sites that rely on the old browser, and users in emerging and underdeveloped markets. There, the global economic recession and sluggish recovery have stalled efforts to upgrade from Windows XP to Windows 7.

"We're working very, very actively [to move IE6's share toward zero], but that takes time," acknowledged Gavin. "We're doing a lot of work with partners and developers on educating users to upgrade to a modern browser," he said, pointing out efforts such as the "Adios IE6" campaign now running in Latin America.

A similar campaign in Australia equated IE6 to a carton of long-expired milk. "You wouldn't drink 9-year-old milk," the campaign's marketing copy read in a page that showed a milk carton dated August 2001, the month when IE6 debuted. "So why use a 9-year-old browser?"

The fifth major browser, Apple's Safari, owned a global usage share of 4.77%, up 0.05 of a percentage point, and a 10.38% share in the U.S.

May's browser usage share data is available on 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 e-mail address is gkeizer@ix.netcom.com .

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This story, "Google's Chrome Browser Is 'in Retreat,' Microsoft Says" was originally published by Computerworld .