Layout Side Panel
The Layout Side Panel gives a quick overview over the box model values of the currently selected node inside the HTML Panel. But it also offers an easy way to change the values by simply clicking on one of them. When hovering over different parts of the box model, the element inside the page highlights, the borders of the hovered part has line guides and a horizontal and vertical rules for measurement.
|Show Rulers and Guides|| ||Specifies whether to display rulers and guides when hovering the parts of the box model|
For more info about tweaks available in Firebug please see the full list of preferences.