Microsoft Accused of Hindering Firefox Browser - WSJ.com

Details tbd, but if Microsoft is planning to block other browsers in Windows RT, I don't think it's about Firefox.  Tangentially, the Apple app model does not preclude using other browsers; e.g., on the iPad, you can download Dolphin and kikin browsers from the iTunes app store.

"Microsoft encourages companies to create apps designed for a new interface it has developed for the operating system, which supports touch-based gestures. But the company doesn't allow developers to also make programs designed to work in a more conventional "desktop" mode for Windows on ARM-based devices, where Microsoft's Internet Explorer is the only browser that will work, Mozilla says.
Harvey Anderson, Mozilla's general counsel, compared the situation to Microsoft actions against browser maker Netscape Communications that helped prompt the U.S. government's antitrust lawsuit against Microsoft in 1998.
"We think it's a dangerous precedent," Mr. Anderson said in a phone interview."

Microsoft Accused of Hindering Firefox Browser - WSJ.com