Re: The problem
NPAPI should be sandboxed (it already is, to a degree). XUL should stay as it allows extremely flexible plugins and XUL apps, if only they actually bothered to document it.
They are jettisoning stuff because they are finding it hard to maintain them (one of their blog entries specifically said this about XUL) but Firefox without NPAPI and XUL is little more than a Chrome-like skin with a different rendering engine. The next step would be to change the rendering engine.
It seems Mozilla are slowly turning into Opera, most of the time they spend on Firefox is futzing around with the UI.