diff --git a/mozilla/content/html/content/src/nsGenericHTMLElement.cpp b/mozilla/content/html/content/src/nsGenericHTMLElement.cpp
index 452c78bc56e..0db0201a5f1 100644
--- a/mozilla/content/html/content/src/nsGenericHTMLElement.cpp
+++ b/mozilla/content/html/content/src/nsGenericHTMLElement.cpp
@@ -425,6 +425,22 @@ nsGenericHTMLElement::GetElementsByTagName(const nsAString& aTagname,
   return nsGenericElement::GetElementsByTagName(tagName, aReturn);
 }
 
+nsresult
+nsGenericHTMLElement::GetElementsByTagNameNS(const nsAString& aNamespaceURI,
+                                             const nsAString& aLocalName,
+                                             nsIDOMNodeList** aReturn)
+{
+  nsAutoString localName(aLocalName);
+
+  // Only lowercase the name if this element has no namespace (i.e.
+  // it's a HTML element, not an XHTML element).
+  if (mNodeInfo && mNodeInfo->NamespaceEquals(kNameSpaceID_None))
+    ToLowerCase(localName);
+
+  return nsGenericElement::GetElementsByTagNameNS(aNamespaceURI, localName,
+                                                  aReturn);
+}
+
 // Implementation for nsIDOMHTMLElement
 nsresult
 nsGenericHTMLElement::GetId(nsAString& aId)
diff --git a/mozilla/content/html/content/src/nsGenericHTMLElement.h b/mozilla/content/html/content/src/nsGenericHTMLElement.h
index a375442547a..3559f0c5168 100644
--- a/mozilla/content/html/content/src/nsGenericHTMLElement.h
+++ b/mozilla/content/html/content/src/nsGenericHTMLElement.h
@@ -108,6 +108,9 @@ public:
   NS_METHOD GetTagName(nsAString& aTagName);
   NS_METHOD GetElementsByTagName(const nsAString& aTagname,
                                  nsIDOMNodeList** aReturn);
+  NS_METHOD GetElementsByTagNameNS(const nsAString& aNamespaceURI,
+                                   const nsAString& aLocalName,
+                                   nsIDOMNodeList** aReturn);
 
   // nsIDOMHTMLElement methods. Note that these are non-virtual
   // methods, implementations are expected to forward calls to these
diff --git a/mozilla/content/html/document/src/nsHTMLDocument.cpp b/mozilla/content/html/document/src/nsHTMLDocument.cpp
index e02aed95f4f..9a34124ab5d 100644
--- a/mozilla/content/html/document/src/nsHTMLDocument.cpp
+++ b/mozilla/content/html/document/src/nsHTMLDocument.cpp
@@ -1395,7 +1395,9 @@ nsHTMLDocument::GetElementsByTagName(const nsAString& aTagname,
                                      nsIDOMNodeList** aReturn)
 {
   nsAutoString tmp(aTagname);
-  ToLowerCase(tmp); // HTML elements are lower case internally.
+  if (!IsXHTML()) {
+    ToLowerCase(tmp); // HTML elements are lower case internally.
+  }
   return nsDocument::GetElementsByTagName(tmp, aReturn);
 }