Script Summary: When active, outlines in red the current DOM element being hovered. The trail of ancestors is shown in a box at the top left.
To enable it, select the User Script Command "Toggle DOM Outliner". This is found by right-clicking the Greasemonkey icon in the status bar or under the Tools > Greasemonkey menu.
To disable it, do that again, or double-click the box at the top left.