In the omni.ja file in Firefox Program Files. Open about:home then right-click and select View Page Source. The path to the aboutHome.xhtml file is in the titlebar.
The problem with editing that file is that every 6 weeks when Firefox is updated you'll probably have to edit that file all over again.

What specifically do you want to tweak?
Additions or changes can usually be accomplished thru a userContent.css file, for most things.
http://kb.mozillazine.org/UserContent.css

For instance - hide the "snippet" and hide the "bar" at the bottom with Downloads, Bookmarks, etc :

@-moz-document url(about:home) {
  #snippetContainer { display:none !important; }
}

@-moz-document url(about:home){
 #launcher { display: none !important; }
}
 

I liked the look of the page and wanted it as a static home page for firefox. But since posting and receiving your reply edmeister I have seen two updates and I think I like how it is dynamic so I will be leaving it as is.

Thanks for getting back to me so quick, pity I couldn't respond as quick as I am still waiting for my Internet to be connected at my new place.

mnemonic

You're welcome.

In what way do you want to tweak or modify the about:home page?