People are voting with their feet (or at least their mouse) for Firefox. The browser has crossed the 20% market share threshold, at least according to stats compiled by Net Applications. Starting the week of October 5, Firefox crossed the 20% mark for the week and has been hovering around that mark since.
Naturally, this is the result across a number of sites -- if you do the stats on a lot of tech sites, Firefox hovers over 50% in many cases. Read Write Web, for example, points to this report and shows that Firefox surpasses 55% market share on that site.
Why does market share matter? Pretty simple -- even though Firefox is easily feature-competitive and just as (if not more) standards compliant as IE, there are still sites coded specifically for IE (though far fewer than a few years ago). Those of us using non-Microsoft platforms need sites that support more than IE, but it's hard to make a case for doing the extra testing if you can't show a sizable user base.
Market share isn't just a popularity contest -- it's a metric that vendors and developers use for choosing the platforms they support.