I was just today trying FF again and discovered vertical tabs was via extension only and holy shit disabling the tabs up top looked like a bit of work. Native vertical tabs and grouping and I'm back. It should just copy the layout that Edge does.
It's not too bad, you change legacy style sheets to true in about:config then put a userChrome.css file in a .mozilla/randomprofilename-default/chrome folder. A lot nicer hiding normal tabs though, I constantly F1 my tab bar anyways: