Though for most of my browsing I use Chrome and IE 10... for some compatibility testing I checked the latest version of Firefox and was amazed by their 3D Rendering Inspector.

Though at first impression it may look like a gimmicky feature .. i think it will save a lot developers time while adjusting and understanding various CSS attributes ..

With WinRT .. hoping VS 11 or IE 10 will incorporate such feature ...

Channel 9 in 3D Rendered Inspector