Narrowing the results to just the United States, and just the Windows operating system the picture shifts some in Microsoft's favor. In the US, across all versions of Windows, Internet Explorer as a whole gained share by 0.19 points to 72.44%, while Chrome and Firefox both declined--by 0.10 and 0.08 percentage points respectively.
To be honest, I think both Microsoft's and Mozilla's views are right. That is one of the funny (awesome, silly, impressive--depends on your perspective) thing about statistics. Was Firefox 4 downloaded more than Internet Explorer 9 on its launch day? Absolutely. Not even Microsoft can deny that. But, with three times the potential pool of PCs, how could it not be?
That isn't a slam of Firefox 4, or an endorsement of IE9. I am simply pointing out the obvious fact that it is difficult, and somewhat silly, to try to compare the two head to head. It's like comparing how many copies Microsoft sells of Office 2010 for Windows vs. Office 2011 for Mac OS X. The Mac OS X audience only makes up about five percent of PCs, compared with nearly 90% for Windows. Of course Office 2010 sells better.
Over time, the numbers could get foggier depending on how you spin them. Mozilla has pledged to pursue a more aggressive cycle of releasing major updates, so Firefox 4 could quickly become Firefox 5, then Firefox 6, Firefox 7, and so on before Microsoft ever gets around to launching Internet Explorer 10. So, Firefox 4 is crushing Internet Explorer 9 today, but when Firefox 5 comes out, that will quickly change.
The more accurate--or more important view--is the long term outlook for the browser as a whole rather than a head to head comparison of specific versions. In that view, Internet Explorer has seen a slow and steady decline--but so has Firefox. The "tortoise" in this race appears to be Chrome--slowly trudging along chipping away market share month by month.
As for the issue of Internet Explorer 9 vs. Firefox 4, I have an appointment scheduled in my calendar to compare notes with Asa after each browser has had a few months, and both vendors go through the process of pushing the browser through update channels. Check back in September and we'll compare notes.