Nenhum deles está disponível, pois o MS não permitirá que aplicativos de terceiros tenham acesso total ao sistema. Esta postagem no blog da Mozilla descreve suas objeções a essa política.
It’s reported that Windows RT (the name Microsoft has given to Windows running on the ARM processor) 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. In practice, this 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.
Does Firefox work on Windows RT?
Firefox is not available for Windows RT.
(Chrome won’t run in WinRT, i.e. Windows 8 on ARM processors, as Microsoft is not allowing browsers other than Internet Explorer on the platform.)