As much as people here aren't a fan of the choice, I saw Firefox OS running on TVs and other IoT devices at this year's MozFest and it was really impressive. The developer APIs were easy to grasp and the UI was intuitive.
What people are implicating is their strategy. As others have mentioned, they don't seem to lead entry into any space, and therefore their presence in the space is constantly in the vein of catching up. Thus far, the only clear reason I've ever heard them mention is the same "open web standards" argument. Obviously this approach is valid, but with their browser success they should know better than anyone that you cannot overtake Goliath with open standards if no one is using your product.
Lo and behold, they've apparently shown a product at MozCon that Google and Apple have already bet a ton on and released compelling consumer offerings for. Regardless of how impressive the APIs and UI may be, history repeating itself here.