Vertical tabs are a game changer. I've been using Vertical Tabs Reloaded [0] which similar but doesn't have nesting and added a userChrome.css in my Firefox profile directory [1] to hide the top bar tabs

  #TabsToolbar {
    visibility: collapse !important;
  }

[0] https://addons.mozilla.org/en-US/firefox/addon/vertical-tabs...

[1] https://www.userchrome.org/how-create-userchrome-css.html