Microsoft dominates the browser "market"
Amount Microsoft makes off of IE (monthly - multiply by 12 for annual amount): $0.00
If FireFox takes over the world and nobody used IE ever again - amount Microsoft will lose: $0.00
The browser is nearly valueless - what is in the browser and what serves up the content for the browser is where the value lies.
Since IE is part of Windows by Microsoft's claim, then the amount of money made off IE is some percentage of the billions of dollars of Windows income.