HTML5 is a classic example Design by committee. We cannot develop for this poor standard with confidence. Rather, one has to do sophisticated browser testing for even the most trivial features. FF OS doesn't any of these issues. All it does is provide another incompatible platform for us to develop for.
As Facebook found out the hard way, HTML5 Apps will always be slower and less usable then their Native counterparts. Targeting Android and iOS covers 90% of the smartphone market and gives us better interoperability and performance to boot.