fxosstub is the most simplistic, minimalist pattern to get the most newbies started building an application for Firefox OS on mobile, and it must stay as such: no rings, no bells. The Ariadne's string through the Labyrinth, just enough to wake up the enthusiasm of legions of developers.
fxosstub does absolutely nothing, except to show a screen suitable for obtaining the familiar layout of a mobile application aujus Firefox: a top header --with an install button, a body section, a bottom toolbar --with normalized icons. In the body section, there is a text area --launching the virtual keyboardy --and a progress bar. All of this is supposed to self-adapt to an adequate size, keeping the text readable, the buttons finger-reachable.
fxosstub fills the gap between (i) what you are used to for building a regular web-page on a laptop/desktop display, and (ii) what is required on mobile with touch screens of reduced size.