Rrreally? I was webdeveloper, and I got sick of the speed of HTML5 development and they way it was done. (not using XML by default is just inexcusable, for example) Also, HTML is general simply sucks or application-like development. It was meant for text, and for that it's fine. But doing easy layout, consistent GUI elements, easy GUI development and interacting with the rest of the platform, that is still a disaster. And no relief in sight.
And low-end devices running Firefox? Hm. Yeah I guess at some point in the future even low-end devices are capable enough, but that point isn't now.
As soon as a server is down the app is non functional.