In late 2012, when I started up the Firefox for Metro team (I know that's not what Microsoft calls it anymore, but it remains how we talk about it in Mozilla), it looked like the next battleground for the Web. Windows is a massive ecosystem and Microsoft pushes its new platforms hard. At first, it looked like we would be locked out completely. We eventually broke open Metro (though never the RT line of ARM-based products) and we got to work.
In the months since, as the team built and tested and refined the product, we've been watching Metro's adoption. From what we can see, it's pretty flat. On any given day we have, for instance, millions of people testing pre-release versions of Firefox desktop, but we've never seen more than 1000 active daily users in the Metro environment.