The thing about that it is that it has really very little to do with Mozilla. ChromeOS is only that because a) ChromeOS and Chrome are both made by Google (there is no "teaming up"), and b) Google's incentive is for everything to be using their services in their browser.
But putting everything in the browser is not generally desirable in its own right. An interesting competitor would have a browser, but it would just be another application. And there have been several attempts at making security-first operating systems, from various Linux distros to OpenBSD, but none of them have Google's resources. And that isn't really a problem Mozilla can solve at this point either.
Why build a second operating system inside the browser instead of just using the original operating system?
If Mozilla teamed up with someone like Ubuntu to produce a viable alternative I'd buy into it fairly quickly honestly.
In the meantime I'm considering an iPad Pro as a "Laptop" alternative and as a way to offload apps from my phone so my phone is just that, a phone with nothing but maybe the work apps on it and texting stuff. I don't want social media in my pocket anymore.
Edit: On another note, maybe if we see more ARM laptops it may be worthwhile for a new Netbook / ARM focused distro to crop up. One thing that kills me about Chromebooks is paying a thousand dollars for decent specs but I could pay the same amount and real a laptop with a real OS on it. The main selling factor with ChromeOS for me though has always been their security model.