Neat, and works well for the toy example with 4 tabs open. I don't really think it would work as well for the "tab collectors" as they think. They seem to be aware of this too, since none of the screenshots in the post have anywhere near what I would consider a significant amount of tabs open.

I use the tree style tab extension on Firefox[0], which I cannot live without. Horizontal tabs become useless after about 15 of them are open. Tree style, 50 tabs are just as easy to navigate as 5. I really wish browsers would build this in as a native feature.

[0]: https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...