Microsoft's Internet Explorer (IE) again lost ground to Apple's Safari and Google's Chrome last month, even as the company launched its newest browser, Web metrics data showed today.

But Microsoft stands behind its decision to limit IE9 to users running Windows Vista and Windows 7.

"It was a very deliberate decision," said Ryan Gavin, senior director of IE, talking about the move to exclude XP users from IE9. "You simply can't build on something that is 10 years ago."

Gavin said that Microsoft "has no second thoughts" about its decision.

According to California-based Net Applications, one of several companies that regularly publishes browser usage data, IE lost nine-tenths of a percentage point of share in March, falling to 55.9%, another record low.

IE9, which debuted more than two weeks ago, accounted for 1% of all browsers, a five-tenths of a point jump over February.

But older editions of IE dropped by more than what IE9 gained.

IE6, the browser Microsoft wants to kill, fell by four-tenths of a point to 11%, while IE7 slipped by two-tenths of a percentage point to 7.9%. And IE8, until last month Microsoft's current browser, dropped half a point to end March at 34.4%.

IE8's slip was the first for that browser since Net Applications began tracking it three years ago, a full year before it shipped in final form in March 2009.

Some rivals, meanwhile, continued to gain share at Microsoft's expense.

Google's Chrome grew its share by six-tenths of a point to account for 11.6% of all browsers used worldwide last month, a record. And Apple's Safari posted a gain of three-tenths of a point to end the month at 6.6%.

Even Mozilla's Firefox, which has lost share eight out of the last 12 months, managed a slight increase of one-tenth of a point, the first increase since December 2010, to account for 21.8% of all browsers.

The March 22 launch of Firefox 4 contributed to Mozilla's small turn-around. Net Applications' statistics show that Firefox 4 boosted its share to 1.7% last month, a 1.1-point increase over February.

Microsoft and Mozilla have each touted the number of downloads of their newest browsers, but the latter has clearly won that battle, claiming 7.1 million downloads on Firefox 4's first day of availability and a record 8.75 million the following day.

Gavin has argued that IE9's numbers should be calculated solely by its use on Vista and Windows 7, and its success or failure judged accordingly.

Net Applications put IE9's share of browsers running on Windows 7 at 3.6%, more than three times the overall average, a fact that Gavin stressed during an interview today. "That's about five times the rate of adoption in a comparable period for IE8," Gavin said.

But Net Applications' data also shows that the newest IE9 rivals -- Chrome 10 and Firefox 4 -- have significant chunks of the Windows 7 browser market. Chrome 10, which Google began pushing to current Chrome users via the browser's silent update mechanism almost a month ago, accounted for 10.2% of Windows 7 browsers in March. Firefox 4, on the other hand, lagged behind IE9 with a 2.8% share on Windows 7.

Ignore the numbers for now, Gavin said earlier this week when he blasted early comparisons as "premature at best, and misleading at worst" because of the differences in the upgrade mechanisms of IE, Chrome and Firefox.

Microsoft plans to add IE9 to Windows Update sometime this month -- today he declined to set a date -- from where it will be offered to Vista and Windows 7 users. Mozilla has yet to offer Firefox 4 to customers running older versions of its browser, but will do that soon, a spokeswoman said Thursday.

The roll-out of IE9 via Windows Update and its Automatics Updates option will wrap up by the end of June, Gavin said today.

While the numbers may not provide an unambiguous case for the success of any of the newest browsers, one thing is clear: Microsoft has bet on IE9 and won't back away from that bet.

"We could have continued down the path we were on," said Gavin, again defending the decision to drop XP from the list of operating systems able to run IE9. "We could have added more features to IE, change the UI, blah, blah, blah. We could have made it work across XP, but that's not what's going to push the Web forward.

"We might have been more cautious [by creating a version of IE9 for XP] but you don't get quantum breakthroughs that way," Gavin said.

Microsoft's taking a risk with this strategy, said Al Hilwa, an analyst with IDC who covers browsers for the research firm. (IDC is owned by IDG, the parent company of Computerworld.)

"[XP] users will have to begin to use other browsers to handle [HTML5 content], and that is a risk because they may elect to stay on the other browser and never come back," said Hilwa in an e-mail reply to questions. "It is basic business that when you open such an opportunity for competitors, it is much harder to win them back. This is particularly true in the kind of fast moving disruptive market we are in and the high quality of the competitive browsers."

Gavin was confident that Microsoft could woo back Windows XP users when they eventually upgraded to Windows 7 or its successor.

"This is a temporal problem," said Gavin, referring to the time it will take XP to disappear. "Either we build a better experience or we don't. Pushing the Web forward, that's the best way to keep users."

Hilwa said that Microsoft made a tough decision to leave Windows XP behind, and that it will take time before anyone, including Microsoft, knows whether that bet paid off.

"I understand the agony on this," said Hilwa. "Microsoft made a complex cost-benefit analysis for supporting XP, and placed their bet. We will have to see how it plays out."

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 March browser statistics can be found on the Net Applications site.