Why is Firefox popular?

We've seen the growth of Firefox as an installed browser, but how does this change the stats? After all - there are a finite number of users. I think it's actually other browsers like Opera that are losing out now - people that had IE and decided to change to another browser (like Opera) are the same people that will initially move to Firefox. In some ways, Microsoft may be glad of the competition, but is it wiping out the diversity in the competition?