On March 26, 2011, IE9 had 1.57 percent market share while Firefox 4 had 3.66 percent market share. In other words, Firefox 4 already has more than double the market share of IE9.
In their respective first 24 hours, IE9 saw 2.35 million downloads and Firefox 4 saw 7.1 million downloads. Download numbers don't necessarily correlate to usage, and while this is certainly true, we now know that Mozilla's latest and greatest is speeding ahead of Microsoft's best browser to date.
Microsoft still has one more trick up its sleeve. The software giant is able to push new versions of IE through the Automatic Updates function of Windows Update. The company will likely see quite a jump in market share when it flips that switch.