Earlier quoted context omitted.
Have you tried out TreeStyle Tabs? It has all of the stacking features I saw in that video, but doesn't have the preview features (maybe you can use it alongside another extension that provides that). https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...
I was about to suggest this too. It's like a more general version of stacking, since you have multiple levels, and much more automatic (but you can drag the tabs into a different tree structure if you want). I think it would solve the GP comment's problem better too. There's not so much need to use multiple windows if you can just use multiple trees in the same window. As a bonus there are many plugins for the plugin…
Even if you do want a new window - drag the common parent tab to a new window and it takes its children along for the ride.