Internet Explorer 9 downloaded 2.3 million times in first day; Firefox 3 managed 8 million!
Of course it isn't an entirely fair comparison. The Internet is a different place today than it was back in 2008. For a start, Google Chrome burst onto the scene and has diluted IE's market share ever since. Then there's the fact that browsers are generally a lot better in 2011 than 2008. When Firefox 3 came out, the only other option was Internet Explorer 7! In contrast, Firefox 3.6, Internet Explorer 8 and Chrome are all good browsers -- in short, there's no compelling reason to upgrade to IE9.
More importantly, let's not forget that the Internet has grown massively in the last three years, gathering hundreds of millions of new users -- and all of those users need Web browsers! It's rather surprising, given the huge target audience, to see Internet Explorer 9 quite so far behind Firefox 3, and even Firefox 3.5. It's not like Microsoft hasn't spent an awful lot on marketing its new browser, and IE9 has received plenty of attention from the tech world, too.
Ultimately, I think, it comes down to the grass roots nature of the open source movement, and the zealous loyalty of Firefox users. Six years ago, millions of people celebrated the release of Firefox 1.0 with launch parties -- including me! -- but no matter how hard Microsoft encourages us to embrace the beauty of the web, I don't think any release of Internet Explorer will ever inspire people to gather in a bar and drink to the long and healthy life of the World Wide Web.