There are 3 main xul files in firebug.
- browserOverlay.xul - overlay of fb to the bottom part of the browser
- firebug.xul - fb window itself when detached into it's own app window
- firebugOverlay.xul - this is shared by both browserOverlay.xul and firebug.xul. You can overlay on top of this file, and changes that you make to this file will show in both fb's browser mode, and detached mode.