My firefox CSS

userChrome.css

@-moz-document url("chrome://browser/content/browser.xhtml") {
/* https://gist.github.com/shvchk/ea637112f88fe29a3f21c141a9fcf709 */
  /* To bring back OS title bar with min/max/close buttons, some users might
   * consider setting browser.tabs.drawInTitlebar to false in about:config
   */
  :root {
    --custom-sidebar-collapsed-width: 35px;
    --custom-sidebar-full-width: 15vw;
    --custom-sidebar-border: 0px solid #404040;
  }

  #TabsToolbar {
    visibility: collapse !important;
  }

  #navigator-toolbox {
    border-bottom: 0 !important;
  }

  #browser {
    position: relative;
  }

  /* Hide header unless hovered over */
  #sidebar-header {
    border: 0 !important;
    padding: 0 !important;
    height: 1px;
    opacity: 0;
  }

  #sidebar-header:hover {
    padding: 8px !important;
    height: auto;
    opacity: 1;
  }
}