Well, Microsoft says that IE7 reached 100 Million Installations. I am not sure how they track it. But it has confirmed dominance of Firefox for sure. Just for record, you could check browser stats published by http://www.w3schools.com/browsers/browsers_stats.asp (not updated till december yet).
IE7 went from 3.1% to 7.1% in November, an increase of 4%.
IE6 went from 54.5% to 49.9%, that is decrease of 4.6%. Assuming that 4% of IE7 increase came from IE6 upgrades, .6% of the users went to some other browser.
Compare that to Firefox, which had its share going up from 28.8% to 29.9%, that is an increase of 1.1% (Which appears to have happend thru that .6% and from few other browsers too).