Firefox has already started developing Firefox Metro style web browser that will work as a desktop application as well as metro application with FireFox 14.  Mozilla is using the Windows runtime C++ Template Library (WRL)  and facing couple problems as there are few restrictions used by Windows 8

“Unfortunately a browser can only participate in Metro mode if it is the default browser. So if Firefox is not the default browser on a system, you can’t use it in Metro mode. This is a decision made by Microsoft.” says Brian R. Bondy in a blog post. 

It is in early stage to but soon we will be able to get more information about it.