Color the gray folder and other container icons in Firefox 56+

color-gray-bookmark-folders.css

  /* Colored bookmark sidebar, toolbar, menu button containers */
    /* Standard folder */
  #bookmarks-view treechildren::-moz-tree-image(container),
  #PlacesToolbarItems toolbarbutton[container="true"] .toolbarbutton-icon,
  #PlacesToolbarItems menu[container="true"] .menu-iconic-left,
  #BMB_bookmarksPopup menu[container="true"] .menu-iconic-icon {
    fill: #e8bb00 !important; /* slightly muted gold */
  }
    /* Live Bookmark (RSS Feed) */
  #bookmarks-view treechildren::-moz-tree-image(container, livemark),
  #PlacesToolbarItems toolbarbutton[container="true"][livemark="true"] .toolbarbutton-icon,
  #PlacesToolbarItems menu[container="true"][livemark="true"] .menu-iconic-left,
  #BMB_bookmarksPopup menu[container="true"][livemark="true"] .menu-iconic-icon {
    fill: orange !important;
  }
    /* Smart bookmark folder */
  #bookmarks-view treechildren::-moz-tree-image(container, query),
  #PlacesToolbarItems toolbarbutton[container="true"][query="true"] .toolbarbutton-icon,
  #PlacesToolbarItems menu[container="true"][query="true"] .menu-iconic-left,
  #BMB_bookmarksPopup menu[container="true"][query="true"] .menu-iconic-icon {
    fill: #69c !important; /* similar to blue smart folder color */
  }
  /* These "containers" are SVG in the sidebar, not yet on the menu */
  #bookmarks-view treechildren::-moz-tree-image(container, OrganizerQuery_BookmarksMenu) {
    fill: olive !important;
  }
  #bookmarks-view treechildren::-moz-tree-image(container, OrganizerQuery_BookmarksToolbar) {
    fill: olive !important;
  }
  #bookmarks-view treechildren::-moz-tree-image(container, OrganizerQuery_UnfiledBookmarks) {
    fill: olive !important;
  }