diff --git a/intl/unicharutil/tools/gencasetable.pl b/intl/unicharutil/tools/gencasetable.pl
index c95d118fac1..f48a3e71b78 100644
--- a/intl/unicharutil/tools/gencasetable.pl
+++ b/intl/unicharutil/tools/gencasetable.pl
@@ -36,6 +36,14 @@
 #
 # ***** END LICENSE BLOCK *****
 
+# If you run this script because the Unicode standard has been updated,
+# check xpcom/string/public/nsCharTraits.h to see whether
+# nsCharTraits<PRUnichar>::ASCIIToLower needs to be updated. It only
+# needs to be update if the Unicode consortium adds (or removes)
+# a Unicode character whose lowercase form is an ASCII character.
+# Currently there are only two such characters: KELVIN SIGN and
+# LATIN CAPITAL LETTER I WITH DOT ABOVE.
+
 ######################################################################
 #
 # Initial global variable
diff --git a/xpcom/string/public/nsCharTraits.h b/xpcom/string/public/nsCharTraits.h
index d5d2ce6b992..e7713c9520f 100644
--- a/xpcom/string/public/nsCharTraits.h
+++ b/xpcom/string/public/nsCharTraits.h
@@ -234,13 +234,27 @@ struct nsCharTraits<PRUnichar>
       }
 
     /**
-     * Convert c to its lower-case form, but only if c is ASCII.
+     * Convert c to its lower-case form, but only if the lower-case form is
+     * ASCII. Otherwise leave it alone.
+     *
+     * There are only two non-ASCII Unicode characters whose lowercase
+     * equivalents are ASCII: KELVIN SIGN and LATIN CAPITAL LETTER I WITH
+     * DOT ABOVE. So it's a simple matter to handle those explicitly.
      */
     static
     char_type
     ASCIIToLower( char_type c )
       {
-        return (c >= 'A' && c <= 'Z') ? (c + ('a' - 'A')) : c;
+        if (c < 0x100)
+          return (c >= 'A' && c <= 'Z') ? c + ('a' - 'A') : c;
+        else
+          {
+            if (c == 0x212A) // KELVIN SIGN
+              return 'k';
+            if (c == 0x0130) // LATIN CAPITAL LETTER I WITH DOT ABOVE
+              return 'i';
+            return c;
+          }
       }
 
     static