I also really liked this hint from their archives:
In Mozilla you can hold down CTRL and click on a table cell on a web page. It will highlight it with a blue border. Keep CTRL down and click around to highlight other cells. Try dragging the mouse around as well. If you click near the top of the table, the entire table is lit up. I find it’s a great way to see where the cell boundaries are on a page. You might use it for debugging your own layouts.