Great add-on. I'm still not a big fan of horizontal tabs but I'm sticking with it until a real a «Tab Groups» alternative comes about. The ones that exists have many problems that need fixing.

Of the "tree tab" add-ons I've tried this one has the most feature sets I'm interested in. Most important >Tab Grouping... Tree hierarchy, Tab Unloading, Searching, Sorting are just really nice features. The latest version can now 'hide' tabs when you switch groups without closing them so now it even more useful. You can switch tab groups and then hide the sidebar if your strapped for screen real-estate.