editing a themes html

Hey guys,

How would I go about editing a theme"s html? I cannot seem to find an html file anywhere in the theme's folder (I'm using blogs MU). There are only php files. I am using firebug to identify elements I want to change and it is displaying an html file to me. I'm using the child theme and know how to edit the CSS but can't get a handle on this html or its whereabouts...

Thanks