Firefox has more than a 4% market share[0][1]. One of the main reasons I've heard, is that it's free, and that the Mozilla Foundation is a non-profit.
I also just found this bit, that explains how some browsers might be over or under estimated, because of how their internals work[2]. So we really don't know what the market shares really are.
Using a Google to check the marketshare of a product rival to Google's Chrome. It's interesting how important Google Search is and how powerful it could be. (BTW I certainly don't imply Google's algorithms are skewed)
It seems their share is shrinking quite fast though.