Apps on the Marketplace are deployed to a device and run locally, offline - complete with an intricate permissions model. The only 'web' aspect is being written in HTML/JS/CSS and the extent to which they integrate with the cloud. No less open than the github hosted, Android app scenario you mention. Maybe Firefox OS does need an f-droid equivalent for a free app catalogue though, if and when Mozilla pull the Marketplace servers down.
So be aware of the distinction between mobile web 'apps' and mobile web sites.