Very good extension, highly customizable.
If you want to hide the legacy tabs toolbar AND the title toolbar too, here are my settings :

* Disable title toolbar

* Tree Style Tab vertigo theme

* In your profile/chrome/userChrome.css file (create it if needed) :

#TabsToolbar, #sidebar-header {
visibility: collapse !important;
}
#TabsToolbar {
margin-bottom: -23px !important;
}
toolbar {
padding-right: 110px !important;
padding-bottom: 2px !important;
padding-top: 2px !important;
}
#navigator-toolbox::after {
border-bottom: 1px solid #474749 !important;
}

I like thin and clean tabs so I added these rules to the Tree Style Tab extra style rules :

* Custom Tree Style Tabs css :

.tab {
height: 25px !important;
border-top: none !important;
border-bottom: dotted 1px #ccc !important;
}

.tab .label {
font-size: 11px !important;
}

And voila, a single bar clean Firefox (tested on developer edition v58).

Just one feature request : a dark (not black) theme using the same grays than Firefox would be great !