I know this is old news by now, but I was not sure how to activate vertical tabs on all of my browsers. It certainly is not obvious, so I wanted to include the instructions here in case anyone else wants to try it out.
Type about:config into the address bar of Firefox. You’ll be greeted with a warning that you are accessing advanced settings - click “Accept the Risk and Continue.” Search for the boolean preference sidebar.verticalTabs and set to true. That’s it! Enjoy your vertical Tabs!
Edit: You may also need to set sidebar.revamp to true if it is not already/automatically set.
Nice, wheres my tab groups?
There is still no built-in solution for nested/tree-like groups, but for creating simple groups Ctrl+N has served me for many years.