Tree Style Tabs has been pretty limited since the port to WebExtensions. It can no longer take the place of the existing tab bar, and instead sits alongside it

Tree Style Tabs has been pretty limited since the port to WebExtensions. It can no longer take the place of the existing tab bar, and instead sits alongside it unless you do some Firefox profile CSS trickery that I never got working properly. Mozilla was considering adding a "hide tab bar" feature but I think they abandoned that.