I used to use tree-style tabs with the normal tab bar hidden. I've moved off Firefox so no guarantee the new big update hasn't broken this, but I used the following userChrome.css (if you're unfamiliar with that then check out https://www.userchrome.org)

    /* Hide tab bar in FF Quantum */
    @-moz-document url("chrome://browser/content/browser.xhtml") {
      #TabsToolbar {
        visibility: collapse !important;
        margin-bottom: 21px !important;
      }

      #sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
        visibility: collapse !important;
      }
    }

I think I probably got it from here: https://www.reddit.com/r/FirefoxCSS/comments/d4hx49/ff69/

Edit: More authoritative source shared in the comments which is probably a better starting point (and has loads of other configuration too): https://github.com/piroor/treestyletab/wiki/Code-snippets-fo...