Using Firebug to tweak CSS, something odd

[AlphaMare]

Hi, everyone.
I am working on a page for a client, who doesn't want me to rewrite his (4 year old) code, just add h1's and alt text etc. for SEO.

This has resulted in my having to modify the CSS, and I have it almost the way he wants it.

However, there were some minor tweaks to be done, to get it exactly the layout he had before, and while using Firebug to tweak the CSS, I am finding something in the CSS panel that I have not seen before.

It shows the CSS I have written, but then further down it shows something like this

html.css (line 124)
           <System>

and it says that it resides in a place called "resource://gre/res/html.css"

I need to alter this to make the page work the way my client wants it to, but I am at a loss as to where this funny "html.css" is!

Is this a browser thing?
Or maybe to do with his page extensions being .aspx?
(I admit, I don't know much about ASP, but should it affect the CSS this much?)

Help!