In the early days of Firefox, the application was limited to the desktops of those who really got personal with their browser. Eventually Firefox's adoption expanded to include a wide range of users, from people who wanted an IE alternative to those who found the extensible functionality of it too good to pass up. In terms of sheer numbers of adoption, however, a huge obstacle for Firefox has been the business desktop. Particularly with companies that have in-house intranets that might rely on custom software. Many of those companies have been locked into or preferred IE.
And yet, that trend is changing. According to recent stats, corporate desktops now have a Firefox propagation as high as 18%. Perhaps still a small share compared to the 25-40% home desktop figures from around the world, but a significant number in and of itself. This shows that larger companies are more willing to try alternative software, and likely are also finding it a better fit for their business than IE. Hopefully, should this trend continue, Mozilla will use that popularity to improve the quality and functionality of their browser. So far, they don't seem to be doing all that bad with the Firefox 3 betas.