userChrome.css

#titlebar,
#toolbar-menubar {
  display: none !important;
}

#TabsToolbar {
  --height: 42px;
  height: var(--height) !important;
  transition: margin-bottom 200ms !important;
  &:not(:has(#tabbrowser-arrowscrollbox tab ~ tab)) /* single tab */ {
    margin-bottom: calc(var(--height) * -1) !important;
    position: fixed !important;
    opacity: 0 !important;
  }
}

#main-window {
  &:not(:has(#tabbrowser-arrowscrollbox tab ~ tab)) /* single tab */ {
    &:not(:has(#sidebar-box:not([hidden]))) /* sidebar hidden */ {
      &:not(:has(#PersonalToolbar:not([collapsed]))) /* bookmarks hidden */ {
        #navigator-toolbox:not(:focus-within) {
          position: fixed !important;
          opacity: 0 !important;
          pointer-events: none !important;
          #urlbar {
            opacity: 0 !important;
          }
        }
      }
    }
  }
}