diff --git a/normalize.css b/normalize.css
index e418de5..bc5d818 100644
--- a/normalize.css
+++ b/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.
  */