It has been out for awhile, but it seems like a lot of people weren't aware of the feature. One additional usage - introduce non-programmers/web designers to what is really behind what they see on a webpage. Most people find the 3D view pretty cool, no matter if they're a technical person or not. It's also been great to see the FF developer tools improve so much recently.

Of course, Firefox 11 was less than 6 months ago.

It was an add-on ("Tilt" iirc) before that too.