Dotzler puts this success down to word of mouth trumping a massive marketing campaign:
As you can see from the chart, Firefox 4's release kicked off with considerably more excitement and momentum. IE 9's release, which was backed by one of Microsoft's biggest launch marketing campaigns in recent history, had a comparatively weak showing.
So what explains this disparity? It's you all. It's every one of you that downloaded Firefox 4, found it to be awesome, and told all of your friends and family about it. No ad campaign from Microsoft can top that. Keep spreading the word. Firefox answers to no one but you!
It's important to bear one thing in mind though. Mozilla pushed Firefox 4 to all users via software update while Microsoft has only so far offered IE9 to those running the beta or RC. All other users have had to seek out the download themselves. Once Microsoft starts to push IE9 via Windows Update to all (no date for this has been announced yet) then I expect that usage share numbers will change. This is a time period that Mozilla should take advantage of.[CORRECTION: Firefox has not begun pushing Firefox 4 via software update to existing users either except to those running the beta and RC.]