Firefox - Hide Title Bar

userChrome.css

.tabbrowser-tab {
    visibility: collapse;
  }
  .titlebar-button {
    height: 27px !important;
  }
  #nav-bar {
    margin-top: -42px;
    margin-right: 140px;
    box-shadow: none !important;
  }
  
  [uidensity="compact"]:root .titlebar-button {
    height: 32px !important;
  }
  [uidensity="compact"]:root #nav-bar {
    margin-top: -32px;
  }
  
  #titlebar-spacer {
    background-color: var(--chrome-secondary-background-color);
  }
  #titlebar-buttonbox-container {
    background-color: var(--chrome-secondary-background-color);
  }
  .titlebar-color {
    background-color: var(--toolbar-bgcolor);
  }
  
  #main-window[inFullscreen="true"] #sidebar-box,
  #main-window[inFullscreen="true"] #sidebar-box + splitter {
      visibility: collapse;
  }
  
  #sidebar-box #sidebar-header {
    display: none !important;
  }