Does anyone know what Firefox had to do to support this in WebExtensions? Is it at all generic, or is it super specific to the Tree Style Tabs extension?
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...
The sidebar API had to be added, but this was added quite a while back (been using it with Tab Center Redux since nightly 56 IIRC).
The tabs API was missing some stuff too. There was a Tree Tabs webext that due to the lack of some tabs api stuff was unable to have full functionality without doing lots of disk I/O in the use case of a large number of tabs, but I think that functionality was since added.
Only missing thing is the ability to hide the tab strip, now (AIUI)
