Microsoft is a known abuser of its monopolistic position which tries to kill competition by various means. Looking at the declining market share of Internet Explorer, Microsoft is planning to block competitors such as Firefox and Chrome from its RT edition of Windows 8. Microsoft calls the ARM version of Windows 8 as RT.
Mozilla claims that "Windows RT will have two environments, a Windows Classic environment and a Metro environment for apps. However, Windows on ARM prohibits any browser except for Internet Explorer from running in the privileged 'Windows Classic' environment."
Mozilla further says, "What it means that only Internet Explorer will be able to perform many of the advanced computing functions vital to modern browsers in terms of speed, stability, and security to which users have grown accustomed. Given that IE can run in Windows on ARM, there is no technical reason to conclude other browsers can’t do the same."
Microsoft's decision to block competitors will raise antitrust concerns. Mozilla warns, " If Windows on ARM is simply another version of Windows on new hardware, it also runs afoul of the EC browser choice commitments and seems to represent the very behavior the DOJ-Microsoft settlement sought to prohibit."