Of course, virtually all of Firefox's gain has come at the expense of IE. Microsoft was initially apathetic about Firefox, but now that their stranglehold on the most common portal to the web has been threatened their tune has changed. If you have paid attention to IE7 and Firefox since the 1.5 release, the speed at which both Microsoft and Mozilla are now trying to develop is pretty interesting. Now, Firefox 3.0 is nearing release, and Firefox 2.0 has seen more than 15 major updates since its release.
I hope the Mozilla team never lets pure numbers of users to be their driving goal, and try to stick to crafting a browser that focuses on quality.