diff --git a/dom/base/Document.cpp b/dom/base/Document.cpp
index 07b390a3fcfe..73445fb4fb2b 100644
--- a/dom/base/Document.cpp
+++ b/dom/base/Document.cpp
@@ -6724,22 +6724,6 @@ Document* Document::GetSubDocumentFor(nsIContent* aContent) const {
   return nullptr;
 }
 
-Element* Document::FindContentForSubDocument(Document* aDocument) const {
-  NS_ENSURE_TRUE(aDocument, nullptr);
-
-  if (!mSubDocuments) {
-    return nullptr;
-  }
-
-  for (auto iter = mSubDocuments->Iter(); !iter.Done(); iter.Next()) {
-    auto entry = static_cast<SubDocMapEntry*>(iter.Get());
-    if (entry->mSubDocument == aDocument) {
-      return entry->mKey;
-    }
-  }
-  return nullptr;
-}
-
 bool Document::IsNodeOfType(uint32_t aFlags) const { return false; }
 
 Element* Document::GetRootElement() const {
diff --git a/dom/base/Document.h b/dom/base/Document.h
index 6ad82c390e98..71328a90197c 100644
--- a/dom/base/Document.h
+++ b/dom/base/Document.h
@@ -1254,11 +1254,6 @@ class Document : public nsINode,
    */
   Document* GetSubDocumentFor(nsIContent* aContent) const;
 
-  /**
-   * Find the content node for which aDocument is a sub document.
-   */
-  Element* FindContentForSubDocument(Document* aDocument) const;
-
   /**
    * Return the doctype for this document.
    */