FirefoxSidebar/extensions/bookmark_arrow.css

/*
  Extension: Bookmark Arrow
  Description: Adds an arrow next to the bookmark folder.
*/

.bookmark-item[container] {
  margin-right: 10px !important;
}

/* Add down arrow to folders on the bookmark bar */
.bookmark-item[container]::after {
  content: "ˇ";
  opacity: 0.55;
  padding-left: 3.5px;
  font-size: 23px;
  display: inline-block;
  position: absolute;
  top: 6.9px;
}