diff --git a/xpcom/glue/nsID.h b/xpcom/glue/nsID.h
index 45f5ca058a5..4da9dc0ac09 100644
--- a/xpcom/glue/nsID.h
+++ b/xpcom/glue/nsID.h
@@ -72,15 +72,18 @@ struct nsID {
    */
 
   inline PRBool Equals(const nsID& other) const {
-#ifdef XP_UNIX
+    // One would think that this could be done faster with a really
+    // efficient implementation of memcmp(), but evidently no
+    // memcmp()'s out there are better than this code.
+    //
+    // See bug http://bugzilla.mozilla.org/show_bug.cgi?id=164580 for
+    // details.
+
     return (PRBool)
       ((((PRUint32*) &m0)[0] == ((PRUint32*) &other.m0)[0]) &&
        (((PRUint32*) &m0)[1] == ((PRUint32*) &other.m0)[1]) &&
        (((PRUint32*) &m0)[2] == ((PRUint32*) &other.m0)[2]) &&
        (((PRUint32*) &m0)[3] == ((PRUint32*) &other.m0)[3]));
-#else
-    return (memcmp(this, &other, sizeof(*this)) == 0);
-#endif
   }
 
   /**