Works for my needs for the most part; it has a vertical view of tabs, which can be grouped into collapseable trees, and moved around, exactly as expected. Some major annoyances though! A known Firefox limitation prevents trees from being created automatically (tabs opened from links can't be automatically opened as children, unless you set ALL TABS to get opened as children which is crazy). The "normal" tab bar can't be hidden. Sync features (i.e. "send tab to device"), bookmark features, and possibly other "normal" tab context-menu features are not integrated into the tree tab sidebar. I'm still playing with it to find all the features but it seems pretty solid so far aside from the above!