3D stack of DOM elements wasn't just a tech demo, it was actually part of the devtools. Removed because no one reimplemented it in a way that's compatible with multiprocess Firefox https://bugzilla.mozilla.org/show_bug.cgi?id=1086492
An amazing feature which was moved to a still accessible extension. Sadly I can't remember the name right now but I found it very recently. Makes understanding HTML nesting rapidly apparent.