I haven't looked closely at the extension but I think the two big building blocks are sidebars [1] and the tabs API [2], both of which have many applications ou

I haven't looked closely at the extension but I think the two big building blocks are sidebars [1] and the tabs API [2], both of which have many applications outside of tree style tabs. The "hide the tabstrip" bug mentioned in other comments here is more narrowly focused than those two but it hasn't landed yet... [1] https://developer.mozilla.org/en-US/Add-ons/WebExtensions/user_interface/Sidebars https://developer.mozilla.org/en-US/Add-ons/WebExtensions/us... [2] https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/tabs https://developer.mozilla.org/en-US/Add-ons/WebExtensions/AP...