It's a fact Firefox has become the most used web browser nowadays. I'm using it right now and there is 79% chances you are using it too, according to my blog statistics. I don't have expressive numbers to show (that's the reason I'm not using Google Advertising yet), but at least 237 visits came from users using Firefox in the last 15 days.
Now Firefow 3 is coming with dramatic improvements, but it is still in beta version, which allows you to navigate comfortably, but not to use all its plug-ins, since they are incompatible with the current implementation.
These days, the Firefox community started a promotion to achieve the greatest number of downloads within 24 hours. The idea is to break the world record and be part of the next edition of the Guinness Book.
Read more about it at http://www.spreadfirefox.com.