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

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

#bookmarks-menu autorepeatbutton,
#personal-bookmarks autorepeatbutton {
  display: none;
}

#bookmarks-menu scrollbox,
#personal-bookmarks scrollbox {
  overflow-y: auto;
}