If they drop below 50%, then they lose a lot of that. And thats worth a lot.
Nonsense. Content is all that matters - not the container for the content.
Netscape controlled the browsers "market" and at one time Netscape had a better product. Microsoft started from the position of 0% and cleaned Netscape's clock but good.
Microsoft makes its money on operating systems, development environments, and services to provide web content (as well as office software). If everybody was using FireFox, Microsoft will make the same amount of money.
Content is king, not the thing that displays the content.
Let's review:
Amount Microsoft made off of IE last year: $0,000.00
Amount FireFox dude will make if every human downloads a copy of FireFox: $0,000.00