Vertical tabs don't really have anything to do with tree-style tabs except a superficial similarity. The entire point is the automatic hierarchical organization.
I think tree-style tabs can be quite useful to those that care to utilize them, but I think the vertical nature is the much better selling point for most people, since its advantages are very easy to grok. I would like to see better integration of addons like it into firefox so it doesn't feel so hacky, though. It would also be nice to be able to toggle between horizontal tabs and vertical ones like in Edge, rather than having to edit css files to get rid of them like you have to in Firefox.
