I'm under the impression that this is the way Firefox's vertical tabs should work.
Certainly, not for novice users, but maybe simply an optional "Advanced Mode" or something similar, that we could just enable from Settings and then have this kind of extremely versatile and powerful tabs, groups, etc., tools at hand. (Tabs unloading!)
Still learning it and day-by-day liking it more and more.
ShyFox theme and Zen Browser seem to use Sidebery natively.
Sidebery is just glorious. <3