I'd advise to leave Firefox (both the OS and other web browsersthe browser) under Super User's umbrella, so to speak.
It's not worth splitting Firefox into a new site, Firefox is a much more finite subject thanI feel the same could be said for Linux or Apple.
Also: Firefox will always be on topic here, so even if there is another Stack Exchange for Firefox OS, there will be no reason to move it unless the original poster specifically asks for it to be moved. Note that Linux and Apple is still on topic at Super User.