gecko-dev/toolkit/themes/shared/notification-popup.inc.css

.popup-notification-panel > .panel-arrowcontainer > .panel-arrowcontent {
  /* In order to display the action buttons near the edge of the arrow panel we
   * have to reset its default padding and specify the padding in the individual
   * "popupnotification" elements instead. To keep the rounded borders of the
   * panel, we also have to ensure the contents are clipped to the border box
   * by hiding the overflow, and we have to override the "display" property so
   * that the height of the contents is computed correctly in that case. */
  padding: 0;
  overflow: hidden;
  display: flex;
  /* Make multiple popupnotifications stack vertically. */
  flex-direction: column;
}