As announced last month, Mozilla has integrated the Tilt 3D extension into the core Firefox code in the latest nightlies, adding yet another improvement to the set of web development tools currently in the pipeline.
As with the Tilt 3D extension, web developers can look at a 3D rendering of a web page and easily see how elements are nested and organized. You can drag the 3D view to rotate it, pan it with the arrow keys, and zoom in and out with the mouse wheel.
Lacking in the implementation so far is a legend, a virtual rotation/panning pad, and even export capabilities found in the original extension.
To access the 3D view, from the Firefox menu, select Web Developer, then Inspect. From the Inspect view, press the 3D button in the lower right area (or press Alt + M hotkey to toggle).
Considering the 6 weeks release cycle, 3D view should become available in final form with Firefox 12, planned for late April.
To try it now, you can get a nightly, or the Tilt 3D extension from Mozilla Add-ons.