However, the restrictions on the sandbox for Metro style apps mean that alternate browsers would either have to be skins around the IE engine, or be very slow (

However, the restrictions on the sandbox for Metro style apps mean that alternate browsers would either have to be skins around the IE engine, or be very slow (because it's not possible to run dynamically compiled aka JITted code). That is why there is a special exemption for "Metro style enabled desktop browsers" (which appear to the user as Metro style apps but have all the capabilities of desktop apps) on Windows 8 (x86), which is what Metro style Firefox will use.