Go to about:config and type groups, it's being actively developed.
There's also another API that you can actually use if you're a developer in Browser Toolbox, enable Browser Toolbox
and type gBrowser.addTabGroup, you can add tabs to group using this API, and if you find a way to append it to Tab Tray you should be able to create something like this image:
This is a feature I want to love so much whenever I've used it in the past. It's great, it's useful, it's super awesome .... then, I just open 10,000 tabs [ok, that's an exaggeration, but I currently have 42 open and that's not even a lot] and they're scattered all over the place anyways and I can't seem to force myself to keep them grouped properly!
To anyone that can get their brain to actually utilize this feature, congrats that it's finally coming to Firefox!
I'm glad that it's coming natively, but hands down there's a whole lot of progress they have yet to make to come close to the usability provided by Sidebery, good that they're also working on native vertical tabs in fact! So I think that if you're looking for a better system you could try out that extension https://addons.mozilla.org/en-US/firefox/addon/sidebery/
The implementation of vertical tabs pales in compared to Sideberry. Nice that it is heading in that direction, though. Plus, there is still Sideberry.