Mozilla vs xulrunner

I was reading about DOM specification and came to know that Mozilla
supports DOM. I read MDN docs and see much of the documentation is
outdated   If I want to use just only DOM  to call some methods  (e.g.
getElementsByTagName) through C++ interface (means XPCOM interface ?)
then what do I need:

Firefox source or xulrunner source ? Do I need to embedd whole 700 MB
mozilla or I can use DOM as a separate component ?