Ok Maybe it is not that simple:
Example: 25% of the visitors we track at Squidoo use Firefox, which is not surprising. But 50% of the people who actually build pages on the site are Firefox users. Twice as many.
This is true of bloggers, of Twitter users, of Flickr users... everywhere you look, if someone is using Firefox, they're way more likely to be using other power tools online. The reasoning: In order to use Firefox, you need to be confident enough to download and use a browser that wasn't the default when you first turned on your computer.
See the full article here: http://sethgodin.typepad.com/seths_blog/2008/04/why-downloading.html