Maemo applications, mobile Nokia n810, for example, have a native file picker dialog that is not the sameas the normal GTK file picker. It is designed for smaller screens. Supporting the Hildon file picker is pretty important for any Mozilla mobile work on that platform, from a widget standpoint.
These patches implemented a feature of opening a local file using the hildon system menu & file chooser widget. But, I'm not sure if this is a right approach because the menu was implemented in a native way instead of using XUL. So we seem to need some mobile specific XUL elements.
This feature has already landed in bug 511290