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.
There is a learning curve, but if that’s something you use often enough to remembers all tge capabilities and quirks then it’s great. Especially combined with a screen zone manager like altsnap and fancyzone