Mozilla just released Firefox 4. What's really cool is that it got over 5 million downloads on the first day. Besides the bandwidth bill, it's impressive because it was over double what Microsoft got with Internet Explorer 9.
IE9 was released but was blocked from installing on Windows XP systems. Since between 40-50% of the world still is using Windows XP, they alienated about half the market. Thanks to that, Firefox has seen new success.
Let's see how Google Chrome's release of their 11th version will turn out.