The American market is Covered
Carlos Slim's "America Movil" has long since signed on to using Firefox OS on their Prepaid device brands that serve the North American market.
United States - TracFone Wireless (TracFone, NET10 Wireless, Straight Talk, SafeLink Wireless, SIMPLE Mobile and Telcel América) 21.337 million
So the statement that there are no carriers signed on in the US isn't exactly correct. Since Prepaid are probably what we'd be expecting for FxOS devices primary market, the US seems fairly well covered. The large carriers will probably follow with higher end devices if the OS catches on beyond the entry level.