Normalize hr

Firefox uses different `box-sizing` and `height` values to all other
browsers. Firefox doesn't currently support `box-sizing` without the
`-moz-` prefix, so we use both the vendor-prefixed and unprefixed
properties to ensure that it matches the `content-box` value of other
browsers. It also requires the `height` to be set to `0`.

Known issue: this does not normalize IE 6/7 as `hr` cannot be reliably
styled in those browsers.

normalize.css

@@ -182,6 +182,17 @@ dfn {
     font-style: italic;
 }
 
+/**
+ * Address differences between Firefox and other browsers.
+ * Known issue: no IE 6/7 normalization.
+ */
+
+hr {
+    -moz-box-sizing: content-box;
+    box-sizing: content-box;
+    height: 0;
+}
+
 /**
  * Address styling not present in IE 6/7/8/9.
  */