[nneonneo]

I use the "Sidebar" style on my Mac, with the TST add-on from piro_or. I previously used a different tree style tab addon but it was not updated for WebExtensions.

TST addon preference style:

    /* Compact tab layout */
    :root { --tab-height: 20px !important; }
    .tab { height: 20px !important; }
    /* Shrink space between pinned tabs and tab bar, only when pins are present */
    #tabbar[style*="margin"] { margin-top: 20px !important; }

userChrome.css:

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

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

Then I needed to turn on the title bar (right click toolbar -> customize -> check title bar at the bottom), and select Density -> Compact. The whole thing ends up looking like this: https://imgur.com/a/c4j9QxR

With some very small tweaks you could change it to look more like a Mac toolbar, I guess, but as it stands it's pretty svelte.