The spread of Firefox has varied largely based upon country, with figures as high as 40% in some European countries but lower than 10% in others. However, the overall trend of Firefox gaining market share worldwide continues, and it seems now they are reaching a point where they will have 20% of the world's Internet-connected desktops using Firefox as their browser. Some may make the choice for technical reasons, some might be jumping on a bandwagon and some might be given it by a friend - no matter how or why they are using it, Mozilla is very pleased with their progress.
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.