Internet Explorer's usage share again dipped last month, even as Microsoft's newest browser, IE9, posted record gains, a Web metrics company said today.

Total IE share fell by eight-tenths of a percentage point in May -- the third consecutive month that Microsoft's browser slid by that amount or more -- to end at 54.3%, a new low for Microsoft. The eight-tenths of a point drop was nearly double the average decline over the last 12 months, hinting that IE's decay has accelerated.

Meanwhile, Mozilla's Firefox remained flat last month, adding less than one-tenth of a percentage point to its share of 21.7%, about the same the open-source browser owned in December 2008.

Both Microsoft and Mozilla debuted new browsers in March: The former launched IE9 in the middle of the month, while the latter shipped Firefox 4 a week later.

Neither has materially helped its maker retain or grow share.

Although IE9's share shot up 1.8 percentage points to 4.2%, that wasn't enough to offset the declines posted by other versions. IE8, the browser bundled with Windows 7, lost 1.8 points on its own -- the third month in a row the venerable browser, and the last that will run on Windows XP, lost ground -- while the older IE7 and the ancient IE6 dropped three-tenths and five-tenths of a point, respectively.

Firefox 4 edged above 10% in May, increasing its share by 4.6 percentage points after Mozilla switched on an update offer to users running the older Firefox 3.5 and 3.6 editions.

The numbers for Mozilla's new browser hint at the possibility that it can turn around the company's share fortunes: Small though it was, May's increase in total Firefox usage was its largest gain in more than a year.

As in the last several months, Google's Chrome and Apple's Safari were the beneficiaries of IE's decline.

Chrome boosted its share by six-tenths of a percentage point to end May with a 12.5% share, accounting for one out of every eight browsers used during the month.

Safari climbed by more than a point to 7.3%, largely on the backs of the iPhone and iPad, which continued to log gains in usage.

Data from StatCounter, an Irish competitor to Net Applications, showed the same broad trends: In its tallies, IE lost share, and Chrome and Safari gained.

Microsoft has aggressively promoted the demise of the 10-year-old IE6 , complete with a special deathwatch site that displays the browser's current share.

The company again boasted today of its success in driving down IE6's number.

"We continue to see positive momentum in people upgrading to a modern browser with the share of IE6 and IE7 worldwide dropping almost another point in May," Roger Capriotti, the director of IE product marketing, said in a Wednesday blog .

As IE executives have for the last several months, Capriotti ignored the overall decline of IE and instead focused on the success of IE9 on Windows 7, the platform Microsoft has said is the only one that matters.

"For Windows 7 customers, the best browser for experiencing [the Web] ... is IE9," said Capriotti.

IE9 has made progress on Windows 7, according to Net Applications: It pegged IE9's share of all browsers running on Windows 7 in May at 12.2%, a 63% increase over the previous month. Net Applications also said that IE9 averaged an even-higher 17% share in the U.S. over the final three days of May.

But other browsers remained more popular on Windows 7, including IE8, with a 42.5% share, Chrome 11 (14.8%) and Firefox 4 (14.1%).

Net Applications calculates browser usage share with data obtained from more than 160 million unique visitors who browse 40,000 Web sites that the company monitors for clients. The May browser statistics can be found on the company's site.