Tree Style Tabs
I use TST extensively to manage hundreds of tabs, and it is a real joy. FWIW, these are the customizations that made TST much more usable for me:<p>1) Very importantly, hide the horizontal tab bar. You get some vertical screen space back, and you aren't distracted by two ways of showing the same tabs. You'll have to edit your userChrome.css (see e.g. <a href="http://kb.mozillazine.org/index.php?title=UserChrome.css&printable=yes" rel="nofollow">http://kb.mozillazine.org/index.php?title=UserChrome.css&pri...</a> for details on how to find it), and add something like this:<p><pre><code>  /* 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;
    }
  }
</code></pre>
There are a few variations; this is the one that works well for me on Mac. The second block also hides the sidebar title for TST, which saves space in the tab bar.<p>2) I customize the tabs themselves to make them more compact, thus letting me see more tabs. This CSS can be placed in the TST addon preferences under the "Advanced" section.<p><pre><code>  /* 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; }

  /* Show title of unread tabs with red font */
  .tab.unread .label {
    color: red !important;
  }

  /* Add private browsing indicator per tab */
  .tab.private-browsing .label:before {
    content: " ";
  }
</code></pre>
I highly recommend this addon, and it's a major differentiator for me between Firefox and Chrome.