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 has showed.

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 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.

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.

"[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. "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."

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.