UserChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

/* hide icons for bookmark folders */
#personal-bookmarks .bookmark-item[container="true"] .toolbarbutton-icon {
 display:none!important;
}
/* center bookmark */
#PlacesToolbarItems {
  display: flex !important;
  justify-content: center !important;
}