Firefox lost about 0.2 of a percentage point last month to end with a usage share of 24.2 percent. 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.
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.
But IE6 continues to get the hook from notable websites. Last week, YouTube announced it would stop supporting the aged browser starting March 13; as of that date, the popular video site won't guarantee that new features will work in the browser.
IE8 captured only a sliver of its siblings' losses: IE8 ended February with a share of 22.5 percent, up just 0.1 of a point, the smallest increase since January 2009, two months before the browser's official release, and at a point when only a release candidate was available to the public.
When NetApplications accounted for IE8's 'compatibility view' - a feature that lets users display sites as rendered by the older, and often web standard-incompatible IE6 and IE7 - Microsoft's newest browser owned a 25.8 percent share, making it the most popular single edition of any browser by a six-point margin.
IE's losses could pick up this month with the broad introduction of a browser choice screen in the European Union beginning today.
The ballot was mandated by an agreement Microsoft reached last year with EU antitrust regulators nearly two years after Norwegian browser maker Opera filed a formal complaint.
The ballot appears on Windows PCs where IE is set as the default browser, and lets users download and install rivals, including Chrome, Firefox, Opera, Safari and others.
Although Firefox has traditionally been the browser to reap the most reward from IE's persistent decline, Chrome now occupies that place.
Over the last 12 months, Chrome has outgained Firefox by more than 2:1, posting an increase of 4.1 points to Firefox's 1.7 points.
If Chrome keeps up the pace of the last three months, it will break the 10% mark in October, 11 months ahead of a goal it set last year.
Net Applications measures browser usage share by collecting systems data from the computers that visit the 40,000 sites it monitors for clients.