Or, as MS said about IE8 in 2009: "LCIE is a collection of internal architecture changes to Internet Explorer that improve the reliability, performance, and scalability of the browser. It also paves the way for future improvements in other areas, including security and usability."
IE6, of course, was designed on a different UI model. Each tab, displayed on the tab bar at the bottom of your screen, was a different process.
IE7, introduced in 2006, followed the more natural MS Windows model demonstrated by FF, and put the tabs at the top of the application window, where Bill Gates and Nature intended them to be. But they followed the FF example of putting all the tabs into one process. They fixed that in 2009, which was 6 years ago.