IE9 is designed to support Vista and Windows 7, while the new Firefox also supports XP as well as Windows 2000. Microsoft has said it decided not to support XP in IE9 because it will utilize the graphics processor, which isn't possible under XP, which was released in 2001. A Microsoft executive told news media that supporting XP would have meant "optimizing for the lowest common denominator."
Like IE, Firefox 4 offers support for hardware acceleration in Vista and Windows 7 by utilizing Direct2D and Direct3D APIs. XP doesn't support Direct2D, but Firefox uses Direct3D on XP to provide partial acceleration in Firefox 4. According to web-statistics firm Net Applications, XP represents 55 percent of operating systems connected to the web, or 61 percent of all Windows-based systems.
Other browsers, including Apple's Safari for Windows, Opera and Google's Chrome, will continue to support XP, which Microsoft has essentially abandoned as it attempts to move users to Vista and Windows 7.