Firefox started work on it's new browser for metro UI over a week ago and it seems the firefox team have set out for the metro browser to be released on firefox 14.
https://wiki.mozilla.org/Windows8

I personally will use IE10 (If all browesers are plug-in free(in metro), how much better could an alternative be for a Metro browser). However, I'm really glad to see that 3rd parties are interested in Windows 8 development, and that Microsoft is working with Mozilla and Google to get Chrome and Firefox on Metro. I think that the less we need to go back to the desktop, the better the experience, and the browser is probably (definintely) the most common application, so this is really a step in the right direction.

ive been using 8 since CP got out.. and im starting to hate it more and more every day....
thinking to swich to vista or 7 (but i cant make a choice)

That's not even what this post is about.

I would say seven is the best OS to use but vista you can pick up next to nothing
but this is going off topic so ask in you own thread.

Ok guys try to stay on topic.

Firefox 14 well its in nightly trunk development right now:

14.0a1 Build 2012 03 22 031220

I've broken up the build string thats what its at right now.

Will probably be getting the metro look in a couple months or more still unknown.