In fact, as a developer, Firefox OS is much more closed to me than Android or iOS are.
The problem with FirefoxOS can be summed up in one word: web. The Mozilla guys have NEVER accepted that the web is a shitty platform. The closest they ever came to that was inventing XUL and XBL, but I read that they have been trying to move away from them for a long time in favour of (gah) HTML 5.
Android and iOS utterly spank FirefoxOS because the engineers who built them have no starry-eyed ideological driven illusions about web technologies. Android manages to be open through the clever trick of .... wait for it .... being open source. Not marriage to a more or less randomly evolved technology stack. iOS simply doesn't care: it's an explicit non-goal for them.