diff --git a/accessible/src/html/HTMLTableAccessible.cpp b/accessible/src/html/HTMLTableAccessible.cpp
index 26962abd6cfd..571989aed8e3 100644
--- a/accessible/src/html/HTMLTableAccessible.cpp
+++ b/accessible/src/html/HTMLTableAccessible.cpp
@@ -959,7 +959,7 @@ HTMLTableAccessible::HasDescendant(const nsAString& aTagName, bool aAllowEmpty)
   nsCOMPtr<nsIDOMElement> tableElt(do_QueryInterface(mContent));
   NS_ENSURE_TRUE(tableElt, false);
 
-  nsCOMPtr<nsIDOMNodeList> nodeList;
+  nsCOMPtr<nsIDOMHTMLCollection> nodeList;
   tableElt->GetElementsByTagName(aTagName, getter_AddRefs(nodeList));
   NS_ENSURE_TRUE(nodeList, false);
 
diff --git a/content/base/public/nsContentUtils.h b/content/base/public/nsContentUtils.h
index 3985bb905a5d..f4365dd90bfb 100644
--- a/content/base/public/nsContentUtils.h
+++ b/content/base/public/nsContentUtils.h
@@ -37,6 +37,7 @@
 #include "nsCharSeparatedTokenizer.h"
 #include "gfxContext.h"
 #include "gfxFont.h"
+#include "nsContentList.h"
 
 #include "mozilla/AutoRestore.h"
 #include "mozilla/GuardObjects.h"
@@ -1814,9 +1815,16 @@ public:
    * Utility method for getElementsByClassName.  aRootNode is the node (either
    * document or element), which getElementsByClassName was called on.
    */
-  static nsresult GetElementsByClassName(nsINode* aRootNode,
-                                         const nsAString& aClasses,
-                                         nsIDOMNodeList** aReturn);
+  static already_AddRefed<nsContentList>
+  GetElementsByClassName(nsINode* aRootNode, const nsAString& aClasses)
+  {
+    NS_PRECONDITION(aRootNode, "Must have root node");
+
+    return NS_GetFuncStringHTMLCollection(aRootNode, MatchClassNames,
+                                          DestroyClassNameArray,
+                                          AllocClassMatchingInfo,
+                                          aClasses);
+  }
 
   /**
    * Returns a presshell for this document, if there is one. This will be
@@ -2180,6 +2188,12 @@ private:
 
   static void DropFragmentParsers();
 
+  static bool MatchClassNames(nsIContent* aContent, int32_t aNamespaceID,
+                              nsIAtom* aAtom, void* aData);
+  static void DestroyClassNameArray(void* aData);
+  static void* AllocClassMatchingInfo(nsINode* aRootNode,
+                                      const nsString* aClasses);
+
   static nsIDOMScriptObjectFactory *sDOMScriptObjectFactory;
 
   static nsIXPConnect *sXPConnect;
diff --git a/content/base/src/nsContentList.cpp b/content/base/src/nsContentList.cpp
index 09d38db88c04..dfd176646530 100644
--- a/content/base/src/nsContentList.cpp
+++ b/content/base/src/nsContentList.cpp
@@ -414,20 +414,9 @@ GetFuncStringContentList(nsINode* aRootNode,
     // we have an entry
     list = new ListType(aRootNode, aFunc, aDestroyFunc, aDataAllocator,
                         aString);
-    if (list && !list->AllocatedData()) {
-      // Failed to allocate the data
-      delete list;
-      list = nullptr;
-    }
-
     if (entry) {
-      if (list)
-        entry->mContentList = list;
-      else
-        PL_DHashTableRawRemove(&gContentListHashTable, entry);
+      entry->mContentList = list;
     }
-
-    NS_ENSURE_TRUE(list, nullptr);
   }
 
   NS_ADDREF(list);
diff --git a/content/base/src/nsContentList.h b/content/base/src/nsContentList.h
index 3bf6f746496a..671c8fe3d98b 100644
--- a/content/base/src/nsContentList.h
+++ b/content/base/src/nsContentList.h
@@ -479,6 +479,7 @@ typedef void* (*nsFuncStringContentListDataAllocator)(nsINode* aRootNode,
                                                       const nsString* aString);
 
 // aDestroyFunc is allowed to be null
+// aDataAllocator must always return a non-null pointer
 class nsCacheableFuncStringContentList : public nsContentList {
 public:
   virtual ~nsCacheableFuncStringContentList();
@@ -488,8 +489,6 @@ public:
       mString == aKey->mString;
   }
 
-  bool AllocatedData() const { return !!mData; }
-
 #ifdef DEBUG
   enum ContentListType {
     eNodeList,
@@ -508,6 +507,7 @@ protected:
     mString(aString)
   {
     mData = (*aDataAllocator)(aRootNode, &mString);
+    MOZ_ASSERT(mData);
   }
 
   virtual void RemoveFromCaches() {
diff --git a/content/base/src/nsContentUtils.cpp b/content/base/src/nsContentUtils.cpp
index 41fc30bbc1e4..f14a3bfef761 100644
--- a/content/base/src/nsContentUtils.cpp
+++ b/content/base/src/nsContentUtils.cpp
@@ -44,7 +44,6 @@
 #include "nsIParser.h"
 #include "nsIFragmentContentSink.h"
 #include "nsIContentSink.h"
-#include "nsContentList.h"
 #include "nsIHTMLDocument.h"
 #include "nsIDOMHTMLFormElement.h"
 #include "nsIDOMHTMLElement.h"
@@ -6245,9 +6244,10 @@ struct ClassMatchingInfo {
   nsCaseTreatment mCaseTreatment;
 };
 
-static bool
-MatchClassNames(nsIContent* aContent, int32_t aNamespaceID, nsIAtom* aAtom,
-                void* aData)
+// static
+bool
+nsContentUtils::MatchClassNames(nsIContent* aContent, int32_t aNamespaceID,
+                                nsIAtom* aAtom, void* aData)
 {
   // We can't match if there are no class names
   const nsAttrValue* classAttr = aContent->GetClasses();
@@ -6273,23 +6273,23 @@ MatchClassNames(nsIContent* aContent, int32_t aNamespaceID, nsIAtom* aAtom,
   return true;
 }
 
-static void
-DestroyClassNameArray(void* aData)
+// static
+void
+nsContentUtils::DestroyClassNameArray(void* aData)
 {
   ClassMatchingInfo* info = static_cast<ClassMatchingInfo*>(aData);
   delete info;
 }
 
-static void*
-AllocClassMatchingInfo(nsINode* aRootNode,
-                       const nsString* aClasses)
+// static
+void*
+nsContentUtils::AllocClassMatchingInfo(nsINode* aRootNode,
+                                       const nsString* aClasses)
 {
   nsAttrValue attrValue;
   attrValue.ParseAtomArray(*aClasses);
   // nsAttrValue::Equals is sensitive to order, so we'll send an array
   ClassMatchingInfo* info = new ClassMatchingInfo;
-  NS_ENSURE_TRUE(info, nullptr);
-
   if (attrValue.Type() == nsAttrValue::eAtomArray) {
     info->mClasses.SwapElements(*(attrValue.GetAtomArrayValue()));
   } else if (attrValue.Type() == nsAttrValue::eAtom) {
@@ -6304,26 +6304,6 @@ AllocClassMatchingInfo(nsINode* aRootNode,
 
 // static
 
-nsresult
-nsContentUtils::GetElementsByClassName(nsINode* aRootNode,
-                                       const nsAString& aClasses,
-                                       nsIDOMNodeList** aReturn)
-{
-  NS_PRECONDITION(aRootNode, "Must have root node");
-  
-  nsContentList* elements =
-    NS_GetFuncStringHTMLCollection(aRootNode, MatchClassNames,
-                                   DestroyClassNameArray,
-                                   AllocClassMatchingInfo,
-                                   aClasses).get();
-  NS_ENSURE_TRUE(elements, NS_ERROR_OUT_OF_MEMORY);
-
-  // Transfer ownership
-  *aReturn = elements;
-
-  return NS_OK;
-}
-
 #ifdef DEBUG
 class DebugWrapperTraversalCallback : public nsCycleCollectionTraversalCallback
 {
diff --git a/content/base/src/nsDocument.cpp b/content/base/src/nsDocument.cpp
index e5f13ce9c0d1..9ff0302d5f75 100644
--- a/content/base/src/nsDocument.cpp
+++ b/content/base/src/nsDocument.cpp
@@ -3034,7 +3034,8 @@ NS_IMETHODIMP
 nsDocument::GetElementsByClassName(const nsAString& aClasses,
                                    nsIDOMNodeList** aReturn)
 {
-  return nsContentUtils::GetElementsByClassName(this, aClasses, aReturn);
+  *aReturn = nsContentUtils::GetElementsByClassName(this, aClasses).get();
+  return NS_OK;
 }
 
 NS_IMETHODIMP
diff --git a/content/base/src/nsGenericElement.cpp b/content/base/src/nsGenericElement.cpp
index a5aa31abb143..7872b1838ac9 100644
--- a/content/base/src/nsGenericElement.cpp
+++ b/content/base/src/nsGenericElement.cpp
@@ -809,9 +809,10 @@ nsGenericElement::GetBoundingClientRect(nsIDOMClientRect** aResult)
 
 NS_IMETHODIMP
 nsGenericElement::GetElementsByClassName(const nsAString& aClasses,
-                                         nsIDOMNodeList** aReturn)
+                                         nsIDOMHTMLCollection** aReturn)
 {
-  return nsContentUtils::GetElementsByClassName(this, aClasses, aReturn);
+  *aReturn = nsContentUtils::GetElementsByClassName(this, aClasses).get();
+  return NS_OK;
 }
 
 NS_IMETHODIMP
@@ -1005,7 +1006,7 @@ nsGenericElement::RemoveAttributeNode(nsIDOMAttr* aAttribute,
 
 nsresult
 nsGenericElement::GetElementsByTagName(const nsAString& aTagname,
-                                       nsIDOMNodeList** aReturn)
+                                       nsIDOMHTMLCollection** aReturn)
 {
   nsContentList *list = NS_GetContentList(this, kNameSpaceID_Unknown, 
                                           aTagname).get();
@@ -1135,7 +1136,7 @@ nsGenericElement::SetAttributeNodeNS(nsIDOMAttr* aNewAttr,
 nsresult
 nsGenericElement::GetElementsByTagNameNS(const nsAString& aNamespaceURI,
                                          const nsAString& aLocalName,
-                                         nsIDOMNodeList** aReturn)
+                                         nsIDOMHTMLCollection** aReturn)
 {
   int32_t nameSpaceId = kNameSpaceID_Wildcard;
 
diff --git a/dom/interfaces/core/nsIDOMElement.idl b/dom/interfaces/core/nsIDOMElement.idl
index 7c7f3b07851d..e3a7d1daf06f 100644
--- a/dom/interfaces/core/nsIDOMElement.idl
+++ b/dom/interfaces/core/nsIDOMElement.idl
@@ -13,7 +13,7 @@
  * http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html#interface-element
  */
 
-[scriptable, uuid(69D44CE2-B544-49A8-BB5F-87804B971EE4)]
+[scriptable, uuid(f1465e67-577f-4546-a7a3-bb1293c0dd15)]
 interface nsIDOMElement : nsIDOMNode
 {
   readonly attribute DOMString        tagName;
@@ -47,14 +47,14 @@ interface nsIDOMElement : nsIDOMNode
   nsIDOMAttr         setAttributeNodeNS(in nsIDOMAttr newAttr)
                                         raises(DOMException);
 
-  nsIDOMNodeList     getElementsByTagName(in DOMString name);
-  nsIDOMNodeList     getElementsByTagNameNS(in DOMString namespaceURI, 
-                                            in DOMString localName);
+  nsIDOMHTMLCollection getElementsByTagName(in DOMString name);
+  nsIDOMHTMLCollection getElementsByTagNameNS(in DOMString namespaceURI, 
+                                              in DOMString localName);
   /**
    * Retrieve elements matching all classes listed in a
    * space-separated string.
    */
-  nsIDOMNodeList getElementsByClassName(in DOMString classes);
+  nsIDOMHTMLCollection getElementsByClassName(in DOMString classes);
 
   /**
    * Returns a live nsIDOMNodeList of the current child elements.
diff --git a/dom/plugins/base/nsPluginInstanceOwner.cpp b/dom/plugins/base/nsPluginInstanceOwner.cpp
index f1a16a004aa3..568405d12472 100644
--- a/dom/plugins/base/nsPluginInstanceOwner.cpp
+++ b/dom/plugins/base/nsPluginInstanceOwner.cpp
@@ -57,6 +57,7 @@ using mozilla::DefaultXDisplay;
 #include "nsIScrollableFrame.h"
 #include "nsIDocShell.h"
 #include "ImageContainer.h"
+#include "nsIDOMHTMLCollection.h"
 
 #include "nsContentCID.h"
 #include "nsWidgetsCID.h"
@@ -1142,7 +1143,7 @@ nsresult nsPluginInstanceOwner::EnsureCachedAttrParamArrays()
   // Making DOM method calls can cause our frame to go away.
   nsCOMPtr<nsIPluginInstanceOwner> kungFuDeathGrip(this);
 
-  nsCOMPtr<nsIDOMNodeList> allParams;
+  nsCOMPtr<nsIDOMHTMLCollection> allParams;
   NS_NAMED_LITERAL_STRING(xhtml_ns, "http://www.w3.org/1999/xhtml");
   mydomElement->GetElementsByTagNameNS(xhtml_ns, NS_LITERAL_STRING("param"),
                                        getter_AddRefs(allParams));
diff --git a/editor/libeditor/text/nsTextEditRules.cpp b/editor/libeditor/text/nsTextEditRules.cpp
index 54354b3e5880..bda1d0f4230c 100644
--- a/editor/libeditor/text/nsTextEditRules.cpp
+++ b/editor/libeditor/text/nsTextEditRules.cpp
@@ -931,7 +931,7 @@ nsTextEditRules::DidRedo(nsISelection *aSelection, nsresult aResult)
     nsCOMPtr<nsIDOMElement> theRoot = do_QueryInterface(mEditor->GetRoot());
     NS_ENSURE_TRUE(theRoot, NS_ERROR_FAILURE);
     
-    nsCOMPtr<nsIDOMNodeList> nodeList;
+    nsCOMPtr<nsIDOMHTMLCollection> nodeList;
     res = theRoot->GetElementsByTagName(NS_LITERAL_STRING("br"),
                                         getter_AddRefs(nodeList));
     NS_ENSURE_SUCCESS(res, res);
@@ -946,10 +946,12 @@ nsTextEditRules::DidRedo(nsISelection *aSelection, nsresult aResult)
         return NS_OK;  
       }
 
-      nsCOMPtr<nsIContent> content = nodeList->GetNodeAt(0);
+      nsCOMPtr<nsIDOMNode> node;
+      nodeList->Item(0, getter_AddRefs(node));
+      nsCOMPtr<nsIContent> content = do_QueryInterface(node);
       MOZ_ASSERT(content);
       if (mEditor->IsMozEditorBogusNode(content)) {
-        mBogusNode = do_QueryInterface(content);
+        mBogusNode = node;
       } else {
         mBogusNode = nullptr;
       }
diff --git a/embedding/components/webbrowserpersist/src/nsWebBrowserPersist.cpp b/embedding/components/webbrowserpersist/src/nsWebBrowserPersist.cpp
index 02caae35d2c7..bb198ad7fe83 100644
--- a/embedding/components/webbrowserpersist/src/nsWebBrowserPersist.cpp
+++ b/embedding/components/webbrowserpersist/src/nsWebBrowserPersist.cpp
@@ -4032,7 +4032,7 @@ nsWebBrowserPersist::SetDocumentBase(
     // Find or create the BASE element
     NS_NAMED_LITERAL_STRING(kBase, "base");
     nsCOMPtr<nsIDOMElement> baseElement;
-    nsCOMPtr<nsIDOMNodeList> baseList;
+    nsCOMPtr<nsIDOMHTMLCollection> baseList;
     if (xmlDoc)
     {
         headElement->GetElementsByTagNameNS(
diff --git a/widget/xpwidgets/GfxInfoBase.cpp b/widget/xpwidgets/GfxInfoBase.cpp
index ef386897e964..b9fb7c1e8390 100644
--- a/widget/xpwidgets/GfxInfoBase.cpp
+++ b/widget/xpwidgets/GfxInfoBase.cpp
@@ -21,6 +21,7 @@
 #include "nsIObserver.h"
 #include "nsIObserverService.h"
 #include "nsIDOMElement.h"
+#include "nsIDOMHTMLCollection.h"
 #include "nsIDOMNode.h"
 #include "nsIDOMNodeList.h"
 #include "nsTArray.h"
@@ -225,7 +226,7 @@ BlacklistOSToOperatingSystem(const nsAString& os)
 }
 
 static GfxDeviceFamily*
-BlacklistDevicesToDeviceFamily(nsIDOMNodeList* aDevices)
+BlacklistDevicesToDeviceFamily(nsIDOMHTMLCollection* aDevices)
 {
   uint32_t length;
   if (NS_FAILED(aDevices->GetLength(&length)))
@@ -323,7 +324,7 @@ BlacklistNodeGetChildByName(nsIDOMElement *element,
                             const nsAString& tagname,
                             nsIDOMNode** firstchild)
 {
-  nsCOMPtr<nsIDOMNodeList> nodelist;
+  nsCOMPtr<nsIDOMHTMLCollection> nodelist;
   if (NS_FAILED(element->GetElementsByTagName(tagname,
                                               getter_AddRefs(nodelist))) ||
       !nodelist) {
@@ -396,7 +397,7 @@ BlacklistEntryToDriverInfo(nsIDOMNode* aBlacklistEntry,
 
       // Get only the <device> nodes, because BlacklistDevicesToDeviceFamily
       // assumes it is passed no other nodes.
-      nsCOMPtr<nsIDOMNodeList> devices;
+      nsCOMPtr<nsIDOMHTMLCollection> devices;
       if (NS_SUCCEEDED(devicesElement->GetElementsByTagName(NS_LITERAL_STRING("device"),
                                                             getter_AddRefs(devices)))) {
         GfxDeviceFamily* deviceIds = BlacklistDevicesToDeviceFamily(devices);
@@ -445,7 +446,7 @@ BlacklistEntryToDriverInfo(nsIDOMNode* aBlacklistEntry,
 }
 
 static void
-BlacklistEntriesToDriverInfo(nsIDOMNodeList* aBlacklistEntries,
+BlacklistEntriesToDriverInfo(nsIDOMHTMLCollection* aBlacklistEntries,
                              nsTArray<GfxDriverInfo>& aDriverInfo)
 {
   uint32_t length;
@@ -476,7 +477,7 @@ GfxInfoBase::Observe(nsISupports* aSubject, const char* aTopic,
   if (strcmp(aTopic, "blocklist-data-gfxItems") == 0) {
     nsCOMPtr<nsIDOMElement> gfxItems = do_QueryInterface(aSubject);
     if (gfxItems) {
-      nsCOMPtr<nsIDOMNodeList> blacklistEntries;
+      nsCOMPtr<nsIDOMHTMLCollection> blacklistEntries;
       if (NS_SUCCEEDED(gfxItems->
             GetElementsByTagName(NS_LITERAL_STRING(BLACKLIST_ENTRY_TAG_NAME),
                                  getter_AddRefs(blacklistEntries))) &&