Today I tried about ten add-ons to replace add-ons Mozilla has killed. This one is the only one that really works very well! With just one click of the mouse you'll get an outline around every element, even with different colors for the outlines. The best part: if you hover over an outline, the outline gets a shadow. So you see in one instance how big the element really is. Incredibly handy with nested lists etc. I can recommend this add-on for everybody working with html! Thanks a lot!