? I don't think this is correct. The version of Firefox that this article is talking about should run on ARM. What you won't get on ARM is a desktop version of Firefox.
No, this version will not run on ARM. This is not a regular Metro app. On x86 systems, Microsoft has made a special exception for third-party browsers to utilize Win32 APIs. So both Chrome and Firefox under x86+Metro are still using their underlying desktop codebase, just with a different UI.
On ARM, only MS apps are allowed to access Win32 APIs.
Aside from legacy code, why do they need the Win32 APIs?
There are also technical limitations on Metro apps. For example, JIT compilation is not possible for pure Metro apps because Metro APIs do not allow marking memory as executable.
It's correct. No other browser will work on Windows RT, just like no other browser will work on WP8.