According to the blog post by Mozilla developer Brian Bondy
This week Mozilla started churning out the code for the ambitious Firefox metro project and lets hope its not ambitious as the shelved/postponed electrolysis, but that was too ambitious.
Mozilla has already developed/optimized Firefox for Android (fennec) for tablets . But as Metro design theme is different to that of android and gives controls to top and the bottom of the app and windows takes the left and right for its own charms bar and apps switching, there will be radical redesign in the firefox metro.
Firefox for Android (fennec)
Windows 8 supports mainly 3 types of applications
1) Classic Desktop applications
2) Metro applications (run in the metro sandbox)
3) Metro style enabled desktop browsers (ability to run outside of the Metro sandbox and have access to the most of Win32 API & WinRT API)
Firefox will fall into the third category Metro enabled desktop browsers , So this fox will have 2 faces one in desktop and other in metro. The metro face will need a lot of new code.
Metro style enabled desktop browsers enjoys the privilege running outside of the Metro sandbox and have access to most Win32 API & WinRT API.So utilizing all these privileges in the metro land of restrictions Mozilla plans to give us a metro style look with the experience of the powerful desktop counterpart.
Firefox Metro mode is only available if Firefox is set as the default browser a decision made by Microsoft.As it is not entirely a metro app and actually a metro style enabled browser The Firefox Metro enabled desktop browser will be distributed in the traditional way and most probably not the app store way.