Does Firefox still have the 3D DOM viewer?

Up until Firefox version 47: If you right click and select "Inspect Element" and then click the gear icon at the left hand side of the toolbox menu you should see "Available Toolbox Buttons" under which appears "3D View".

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Clicking this will add a new icon to the toolbox menu that when clicked will show the 3D Dom view.

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From Firefox 47 onwards, built-in 3D view is no longer available.

There is an add-on that offers the same functionality: Tilt 3D.

Note: just like the built-in version, the add-on does not work in multiprocess Firefox.

Tilt 3D IS Not compatible with Firefox Quantum.

Goto: Preferences->General Uncheck "Enable multi-process Firefox Developer Edition"

Restart Firefox

Goto: Developer tool bar [Tool Box Options], "3D View" check box should now show under "Available Toolbox Buttons", make sure this is checked and you are good.