Microsoft's Internet Explorer, once running on nearly 90 percent of computers, is now used by just two out of every three web users, according to Net Applications.
The web metrics firm said 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 release 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.
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 ASA's flagship browser remained stuck at 0.7 percent, where it has languished for nearly a year.