there is a key difference: it does not lock you into Mozilla or lock you into your Firefox OS phone
How is this true in any meaningful way? I look down this list of APIs: https://wiki.mozilla.org/WebAPI and I see many many things that are FirefoxOS only, and are not planned to be made available for other platforms.
If they want to say that FirefoxOS can display 'standards compliant' web pages then yes, great, but so can every other OS to some degree (web 'standards' are a moving target and Chrome and Safari have broadly comparable standards compliance).
If FirefoxOS is to be truly standards compliant that means it will be unable to do a single thing that other platforms can't also do in their respective standards compliant web browsers (and if no other browser implements it then it's not much of a standard). So where is the additional value from FirefoxOS? It's just an OS that is by definition less functional than every other OS.
Before anybody chimes in pointing out all the misleading PR from Apple and Google and Microsoft, yes I'm aware of that, but they are for profit companies and don't claim to be working for the betterment of mankind. If to compete with them you also have to coat everything in a glaze of PR bullshit then how are you any better than them, other than that your bullshit has a slightly different consistency that makes it easier to swallow for some people.