It's the default style sheet of the browser: User Agent CSS. Your CSS is called Author CSS. It's the one applied by default, that rules h1 be in bigger font than h6, p, div to be block level elements, a, img to be inline level elements, a link be blue, visited be magenta, etcetera.
Underneath html.css you have written in red: <System>. You can hide/show this default style clicking Show User Agent CSS in the Style menu for the Firebug.
If you want to take a look at it, it's found in the omni.jar file in the installation folder, under the \chrome\toolkit\res path. Or just click on it in the Firebug window.