As I was watching the video I was thinking the same thing that WebOS has been out for years. I'm sure the underlying technology and framework is quite different from Mozilla, but it's not a ground-breaking idea.
Perhaps they should take the opportunity to work on the newly open sources WebOS which is likely to be able to grab more market share than an entirely new Gecko OS.
WebOS uses JS+HTML but it does not expose nor advance standards-based APIs for all the phone hardware capabilities like B2G. Also, WebOS requires you use their framework where B2G lets you use any frameworks or no frameworks -- it's really just like the real Web plus a bunch of DOM APIs for things like the phone's telephony stack and vibrator and accelerometer and all that.
Perhaps they should take the opportunity to work on the newly open sources WebOS which is likely to be able to grab more market share than an entirely new Gecko OS.