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

#contentAreaContextMenu,
#contentAreaContextMenu menupopup {
  max-height: 300px; /* or any height you specify */
}

#contentAreaContextMenu autorepeatbutton {
  display: none;
}

#contentAreaContextMenu scrollbox {
  overflow-y: auto;
}