The problem is only in Windows RT i.e. Windows 8 running on ARM hardware. In this configuration there is still WinRT/Metro but there is no desktop. What there is amounts to a sort of hacked together simulated classic desktop that supports some of the API. Microsoft has made version of Office and IE for this environment but wont share the API with other programmers.
Mozilla has now realized, blog post that this means that it can create Firefox for the Metro environment but not for the "classic" desktop on ARM — that's just for IE. As they point out this return to the bad old days could be against the law however as things stand at the moment the situation is far worse than the picture that Mozilla paints. It isn't just browsers that are unable to run under Windows RT classic. It seems that a port of LibreOffice say to compete against Microsoft Office on Windows RT would also be next to impossible.
Currently the Windows RT classic desktop environment is open only to Microsoft.
It is well past the time to get upset about this.