What Mozilla certainly don't want is marketing BS confusing users into thinking IE9 is modern, while it holds the web back and drags its feet about updates. Nor does it want any browser controlled by Microsoft to have dominant market share while Microsoft still sees the web as a threat that it can fight mostly by go-slow tactics like having IE6-8 hanging around preventing new web technologies hitting critical mass.
They don't even have spellchecking in their browser because the Word guys feel threatened.