This the one, the only 10-star add-on.
5 stars for the value of its mission
5 stars for execution
Firefox has some work to catch up to where it was before, but it can still handle this add-on quite well. Visual, collapsible tab tree just makes the browsing experience so much more sensible.
I'm looking forward to seeing the weird old tab bar disappear again, but that's a problem with WE not with this add-on.
I can hardly remember a time I was this happy with a browser feature since the invention of the DOM inspector.