According to the most recent data made available by the market research firm today, Internet Explorer stands at 69.88% today, while Firefox 2 and 3 account for a combined market share of 20.68% (Firefox 2: 13.75%, Firefox 3: 6.92%.)
I just love stories like this. If you would have walked Sand Hill Road in 2002/03/04 pitching a new browser start-up, you would have been laughed out of every VC's conference in record time. The browser wars were over. Microsoft won. There chief competitor, Netscape, was cooked and given the locked in distribution IE had, no one could take market share.
So how did Firefox get 20% of the market then? It's not like using Firefox is a passive opportunity - it doesn't just show up on your machine. You actually have to actively seek it out by going to mozilla.com and downloading it. How on earth did they get so many people to do that given that every machine bought does come pre-loaded with a working alternative?
Lots of people could probably give you lots of reasons - feature set (the tabs!), Mozilla's open source mojo within the developer community, affiliation with Google, etc. I have my theories, but in truth its probably a bit of all of those things.
The point is that there is no such thing as a dead market. Every day new entrants manage to cleverly find ways to wrangle market share away from competitors in very mature markets. Every one of these markets has lots of barriers to entry: start-up costs, distribution difficulties, marketing challenges, pricing problems, etc. But every day, smart companies figure out a way to get through all of these and steal market share. Firefox is just the latest example.
One more note, Apple Safari is not mentioned in this article, so either they're not being counted or most Mac users are also Firefox users. Either way, as Mac continues to grow its laptop market share, that can't bode well for IE either.