Life is getting increasingly tough for Microsoft's Internet Explorer, which four years ago was running on nearly nine out of 10 computers. As of last month, just two out of every three users surfed the Internet using IE, a web metrics company has found.

IE lost 0.7 of a percentage point to end March with a 66.8 percent share of the browser market, the lowest number since Net Applications began tracking browsers in 2005. The launch of IE8 two weeks ago didn't stop or even slow Microsoft's slide; the browser's March drop was slightly larger than the average loss during the previous 12 months.

Earlier data from NetApp indicated that in IE8's first full week of availability, users of rival browsers weren't persuaded to switch. The Microsoft browser was instead downloaded and installed by people who had been running IE7.

In the past year, IE's share has slipped 8 percentage points. If the current rate of decline continues, Microsoft's share will fall below 60 percent in January 2010, the company's publicly stated delivery date for the Windows 7 operating system.

As has generally been the pattern, IE's March losses were countered by rival browsers' gains. Mozilla's Firefox increased its share by the largest amount, 0.3 percentage points, while Apple's Safari and Google's Chrome grew by 0.2 and 0.08 percentage points, respectively.

Firefox, which as of March had six consecutive months of growth, ended March with 22 percent of the browser market, a record for the open-source browser. The beta of Firefox 3.5 - numbered 3.1 until a recent name change by Mozilla - accounted for about 6 percent of all Firefox browsers in use last month, more than double the percentage of IE users running the now-finished IE8.

Safari, meanwhile, returned to the black in March after losing share the month before. And with 8.2 percent of the total market, it has nearly returned to its January 2009 record of 8.3 percent.

Google's Chrome picked up some users as well, ending the month at 1.2 percent, but Opera Software's flagship browser remained stuck at 0.7 percent, where it has languished for nearly a year.

NetApp measures browser usage by tracking the machines that visit the 40,000 sites it monitors for clients.

The current browser share data is available online.