diff --git a/dom/xul/XULDocument.h b/dom/xul/XULDocument.h
index 00d4b497f5ca..e8af3c55d26a 100644
--- a/dom/xul/XULDocument.h
+++ b/dom/xul/XULDocument.h
@@ -150,7 +150,7 @@ public:
     virtual nsIDocument::DocumentTheme GetDocumentLWTheme() override;
     virtual nsIDocument::DocumentTheme ThreadSafeGetDocumentLWTheme() const override;
 
-    virtual void ResetDocumentLWTheme() override { mDocLWTheme = Doc_Theme_Uninitialized; }
+    void ResetDocumentLWTheme() { mDocLWTheme = Doc_Theme_Uninitialized; }
 
     NS_IMETHOD OnScriptCompileComplete(JSScript* aScript, nsresult aStatus) override;
 
diff --git a/dom/xul/nsIXULDocument.h b/dom/xul/nsIXULDocument.h
index 6ad6c7328e24..2ce6f0f8b731 100644
--- a/dom/xul/nsIXULDocument.h
+++ b/dom/xul/nsIXULDocument.h
@@ -34,8 +34,6 @@ class nsIXULDocument : public nsISupports
 {
 public:
   NS_DECLARE_STATIC_IID_ACCESSOR(NS_IXULDOCUMENT_IID)
-
-  virtual void ResetDocumentLWTheme() = 0;
 };
 
 NS_DEFINE_STATIC_IID_ACCESSOR(nsIXULDocument, NS_IXULDOCUMENT_IID)
diff --git a/dom/xul/nsXULElement.cpp b/dom/xul/nsXULElement.cpp
index 2dca5b14e6aa..d6a8ff3776b0 100644
--- a/dom/xul/nsXULElement.cpp
+++ b/dom/xul/nsXULElement.cpp
@@ -40,7 +40,6 @@
 #include "nsIURL.h"
 #include "nsViewManager.h"
 #include "nsIWidget.h"
-#include "nsIXULDocument.h"
 #include "nsLayoutCID.h"
 #include "nsContentCID.h"
 #include "mozilla/dom/Event.h"
@@ -1161,7 +1160,7 @@ nsXULElement::AfterSetAttr(int32_t aNamespaceID, nsAtom* aName,
                 } else if (aName == nsGkAtoms::lwtheme ||
                          aName == nsGkAtoms::lwthemetextcolor) {
                     // if the lwtheme changed, make sure to reset the document lwtheme cache
-                    nsCOMPtr<nsIXULDocument> xuldoc = do_QueryInterface(document);
+                    XULDocument* xuldoc = document->AsXULDocument();
                     if (xuldoc) {
                         xuldoc->ResetDocumentLWTheme();
                         UpdateBrightTitlebarForeground(document);
@@ -1198,7 +1197,7 @@ nsXULElement::AfterSetAttr(int32_t aNamespaceID, nsAtom* aName,
                 } else if ((aName == nsGkAtoms::lwtheme ||
                             aName == nsGkAtoms::lwthemetextcolor)) {
                     // if the lwtheme changed, make sure to restyle appropriately
-                    nsCOMPtr<nsIXULDocument> xuldoc = do_QueryInterface(doc);
+                    XULDocument* xuldoc = doc->AsXULDocument();
                     if (xuldoc) {
                         xuldoc->ResetDocumentLWTheme();
                         UpdateBrightTitlebarForeground(doc);