I am a bit late in finding this out but I just read that, on Feb 21st 2008, Firefox reached a whopping 500 million downloads. Now that really is a marvellous achievement. I remember those days (years ago) when IE was the popular browser and everyone used to code around compatibility between different versions of IE, but with browsers like Firefox around, things really have turned around. I myself have stopped using IE completely now (unless I want to test how a website looks in IE).
The full details can be found here.