Today 10 years ago I went to Poland to buy a Phone with pre installed #Firefox OS on. The Phone was a Alcatel One, so very shitty. Two years later I installed Firefox OS on my Nexus 5 instead.
It was a very good concept, but sadly rolled out on too shitty hardware so it never caught on.
If you mean the OS, it can be purchased in all of European Union plus some other countries. With VPN you can purchase it everywhere. Though I read that the Xperia phones don't have the necessary bands for USA.