If a browser does not support Metro, it is seriously at risk of losing the default browser status, and therefore significant market share. A browser without support for Metro, if default, would be taking away a Metro browser completely from the user’s computer. Even if a user spends most of their time in the Desktop interface, having a really good Metro browser may be enough for the user to change their default browser. A browser with great Metro support can gain significant browser market share for this reason. It is extremely important that we deliver an awesome Firefox experience on Metro, one that is tightly integrated with the platform, fast, and feature rich. Windows is by far the platform with the most users and which has the biggest effect on market share.
Firefox Windows 8 Status Update via Brian R. Bondy.