diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index 47601aabb0..99452df650 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -243,12 +243,11 @@ CustomElementRegistry::~CustomElementRegistry()
 }
 
 CustomElementDefinition*
-CustomElementRegistry::LookupCustomElementDefinition(const nsAString& aLocalName,
+CustomElementRegistry::LookupCustomElementDefinition(nsIAtom* aNameAtom,
                                                      nsIAtom* aTypeAtom) const
 {
-  nsCOMPtr<nsIAtom> localNameAtom = NS_Atomize(aLocalName);
   CustomElementDefinition* data = mCustomDefinitions.GetWeak(aTypeAtom);
-  if (data && data->mLocalName == localNameAtom) {
+  if (data && data->mLocalName == aNameAtom) {
     return data;
   }
 
diff --git a/dom/base/CustomElementRegistry.h b/dom/base/CustomElementRegistry.h
index 01c32a5958..c416e50439 100644
--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -412,7 +412,7 @@ public:
    * https://html.spec.whatwg.org/#look-up-a-custom-element-definition
    */
   CustomElementDefinition* LookupCustomElementDefinition(
-    const nsAString& aLocalName, nsIAtom* aTypeAtom) const;
+    nsIAtom* aNameAtom, nsIAtom* aTypeAtom) const;
 
   CustomElementDefinition* LookupCustomElementDefinition(
     JSContext* aCx, JSObject *aConstructor) const;
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 402dfd1c5b..b6cbbbacef 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -9536,9 +9536,11 @@ nsContentUtils::TryToUpgradeElement(Element* aElement)
   NodeInfo* nodeInfo = aElement->NodeInfo();
   RefPtr<nsIAtom> typeAtom =
     aElement->GetCustomElementData()->GetCustomElementType();
+
+  MOZ_ASSERT(nodeInfo->NameAtom()->Equals(nodeInfo->LocalName()));
   CustomElementDefinition* definition =
     nsContentUtils::LookupCustomElementDefinition(nodeInfo->GetDocument(),
-                                                  nodeInfo->LocalName(),
+                                                  nodeInfo->NameAtom(),
                                                   nodeInfo->NamespaceID(),
                                                   typeAtom);
   if (definition) {
@@ -9553,7 +9555,7 @@ nsContentUtils::TryToUpgradeElement(Element* aElement)
 
 /* static */ CustomElementDefinition*
 nsContentUtils::LookupCustomElementDefinition(nsIDocument* aDoc,
-                                              const nsAString& aLocalName,
+                                              nsIAtom* aNameAtom,
                                               uint32_t aNameSpaceID,
                                               nsIAtom* aTypeAtom)
 {
@@ -9577,7 +9579,7 @@ nsContentUtils::LookupCustomElementDefinition(nsIDocument* aDoc,
     return nullptr;
   }
 
-  return registry->LookupCustomElementDefinition(aLocalName, aTypeAtom);
+  return registry->LookupCustomElementDefinition(aNameAtom, aTypeAtom);
 }
 
 /* static */ void
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 4200a06213..bf6a59dcd9 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -2712,7 +2712,7 @@ public:
    */
   static mozilla::dom::CustomElementDefinition*
     LookupCustomElementDefinition(nsIDocument* aDoc,
-                                  const nsAString& aLocalName,
+                                  nsIAtom* aNameAtom,
                                   uint32_t aNameSpaceID,
                                   nsIAtom* aTypeAtom);
 
diff --git a/dom/base/nsNodeUtils.cpp b/dom/base/nsNodeUtils.cpp
index f79da66523..384e56cde6 100644
--- a/dom/base/nsNodeUtils.cpp
+++ b/dom/base/nsNodeUtils.cpp
@@ -497,9 +497,11 @@ nsNodeUtils::CloneAndAdopt(nsINode *aNode, bool aClone, bool aDeep,
       if (data || !extension.IsEmpty()) {
         RefPtr<nsIAtom> typeAtom = extension.IsEmpty() ? tagAtom : NS_Atomize(extension);
         cloneElem->SetCustomElementData(new CustomElementData(typeAtom));
+
+        MOZ_ASSERT(nodeInfo->NameAtom()->Equals(nodeInfo->LocalName()));
         CustomElementDefinition* definition =
           nsContentUtils::LookupCustomElementDefinition(nodeInfo->GetDocument(),
-                                                        nodeInfo->LocalName(),
+                                                        nodeInfo->NameAtom(),
                                                         nodeInfo->NamespaceID(),
                                                         typeAtom);
         if (definition) {
diff --git a/dom/html/nsHTMLContentSink.cpp b/dom/html/nsHTMLContentSink.cpp
index 920ded728d..1fe5d2a865 100644
--- a/dom/html/nsHTMLContentSink.cpp
+++ b/dom/html/nsHTMLContentSink.cpp
@@ -287,9 +287,10 @@ NS_NewHTMLElement(Element** aResult, already_AddRefed<mozilla::dom::NodeInfo>&&
   CustomElementDefinition* definition = aDefinition;
   if (CustomElementRegistry::IsCustomElementEnabled() && isCustomElement &&
       !definition) {
+    MOZ_ASSERT(nodeInfo->NameAtom()->Equals(nodeInfo->LocalName()));
     definition =
       nsContentUtils::LookupCustomElementDefinition(nodeInfo->GetDocument(),
-                                                    nodeInfo->LocalName(),
+                                                    nodeInfo->NameAtom(),
                                                     nodeInfo->NamespaceID(),
                                                     typeAtom);
   }
diff --git a/parser/html/nsHtml5TreeOperation.cpp b/parser/html/nsHtml5TreeOperation.cpp
index d747f80a8a..22c8058595 100644
--- a/parser/html/nsHtml5TreeOperation.cpp
+++ b/parser/html/nsHtml5TreeOperation.cpp
@@ -433,8 +433,9 @@ nsHtml5TreeOperation::CreateHTMLElement(
       RefPtr<nsIAtom> typeAtom =
         isValue.IsEmpty() ? tagAtom : NS_Atomize(isValue);
 
+      MOZ_ASSERT(nodeInfo->NameAtom()->Equals(nodeInfo->LocalName()));
       definition = nsContentUtils::LookupCustomElementDefinition(document,
-        nodeInfo->LocalName(), nodeInfo->NamespaceID(), typeAtom);
+        nodeInfo->NameAtom(), nodeInfo->NamespaceID(), typeAtom);
 
       if (definition) {
         willExecuteScript = true;