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

/* To hide an extension from Extensions Manager: */

#extensionsManager extension[name="Extension Name"],
#extensionsManager richlistitem[name="Extension Name"] {
  display: none !important;
}


/* To hide ALL extensions from Extensions Manager: */

#extensionsManager extension,
#extensionsManager richlistitem {
  display: none !important;
}