All depends on the carriers pushing the phone. They may say they support the device, but if they give it no visibility, no one will buy it. Even if there is visibility, the carriers need to actively sell the phone in the store. The nokia windows device is a great example of carriers taking the marketing money and not 'selling' the phone because their sales rep would rather push an Android or iPhone to you instead.

Conversely, compare that to Bada (Samsung device) which sold 20M units last year, good carrier relations go a long way.

In this respect, I don't thinik carriers care all that much about Firefox OS, sure they will support them, but they are more likely to support Tizen with their marketing dollars because - a) Previous good press from the Nokia N9 which Tizen was borne out of b) Samsung are a excellent counterweight to Apple and can supply the marketing dollars to back it c) Intel iare helping to push Tizen d) No major conflicts of interest with carriers (e.g. Microsoft own skype which is a conflict, Apple directly updates software.. something carriers don't like)