Google's Chrome, meanwhile, continued its climb for the 16th straight month.
Firefox lost about 0.2 of a percentage point last month to end with a usage share of 24.2 percent. It was the third month in a row that Mozilla's browser fell in the rankings posted by Web measurement vendor NetApplications.com. Previous dips in Mozilla's share were one-month blips; this is the browser's first sustained decline in NetApplications' numbers.
Since the end of November 2009, Firefox has dropped nearly half a point, representing a fall of 2 percent of its total share.
NetApplications is not the only metrics company to note Firefox's slump. The most recent data from Irish vendor StatCounter, for example, shows an even larger drop of 1.2 points -- or 3.6 percent of its total share -- since November.
The slide came even as Mozilla converted a sizable chunk of its Firefox 3.5 user base to the newer Firefox 3.6, the upgrade that shipped in late January . Last month, Firefox 3.6's share surged by nearly 4 percentage points to 5.1 percent. Meanwhile, Firefox 3.5's share fell 2.5 points to 14.6 percent, and the even older Firefox 3.0 dipped by 1.6 points to 3.6 percent, its lowest share since the month it was released in 2008.
As Firefox slid, Google's Chrome again boosted its share, although the increase was smaller than in the two months before. Chrome ended February with a 5.6 percent share, up 0.4 of a percentage point. Chrome has doubled its share in the last six months.
Internet Explorer (IE) lost a half percentage point last month to finish with a share of 61.2 percent, yet another record low for Microsoft 's browser. The upside is that IE's decline in February was under its 12-month average drop of 0.6 of a percentage point, and the smallest in a year. Microsoft may, in fact, find some comfort in NetApplications' numbers: IE's slide may be slowing. In the last three months, IE lost 2 percentage points, while during the three-month period immediately before that the browser slipped by 3.4 points.
Apple 's Safari posted a slight decline to 4.4 percent for February, while Opera Software's desktop browser essentially remained flat at 2.4 percent.
Not surprisingly, Microsoft's nearly-nine-year-old IE6 again fell in NetApplications' usage measurements, ending last month at 19.8 percent, down 0.2 of a percentage point. The newer IE7 lost more than four times more share, falling 0.9 of a point to 13.6 percent. IE6's decline was well under its torrid pace of the previous four months, when it lost more than a point per month.