--- a/dom/interfaces/css/moz.build
+++ b/dom/interfaces/css/moz.build
@@ -4,17 +4,16 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 with Files("**"):
     BUG_COMPONENT = ("Core", "DOM: CSS Object Model")
 
 XPIDL_SOURCES += [
     'nsIDOMCounter.idl',
-    'nsIDOMCSSFontFaceRule.idl',
     'nsIDOMCSSFontFeatureValuesRule.idl',
     'nsIDOMCSSImportRule.idl',
     'nsIDOMCSSKeyframeRule.idl',
     'nsIDOMCSSKeyframesRule.idl',
     'nsIDOMCSSPageRule.idl',
     'nsIDOMCSSPrimitiveValue.idl',
     'nsIDOMCSSRule.idl',
     'nsIDOMCSSRuleList.idl',

deleted file mode 100644
--- a/dom/interfaces/css/nsIDOMCSSFontFaceRule.idl
+++ /dev/null
@@ -1,14 +0,0 @@
-/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-#include "nsISupports.idl"
-
-interface nsIDOMCSSStyleDeclaration;
-
-[scriptable, uuid(db971017-fe0c-4529-972c-8217f2fee217)]
-interface nsIDOMCSSFontFaceRule : nsISupports
-{
-  readonly attribute nsIDOMCSSStyleDeclaration  style;
-};

--- a/layout/style/nsCSSFontFaceRule.cpp
+++ b/layout/style/nsCSSFontFaceRule.cpp
@@ -321,19 +321,16 @@ nsCSSFontFaceStyleDecl::WrapObject(JSCon
 
 /* virtual */ already_AddRefed<css::Rule>
 nsCSSFontFaceRule::Clone() const
 {
   RefPtr<css::Rule> clone = new nsCSSFontFaceRule(*this);
   return clone.forget();
 }
 
-NS_IMPL_ADDREF_INHERITED(nsCSSFontFaceRule, mozilla::css::Rule)
-NS_IMPL_RELEASE_INHERITED(nsCSSFontFaceRule, mozilla::css::Rule)
-
 NS_IMPL_CYCLE_COLLECTION_CLASS(nsCSSFontFaceRule)
 
 NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN_INHERITED(nsCSSFontFaceRule,
                                                mozilla::css::Rule)
   // Keep this in sync with IsCCLeaf.
 
   // Trace the wrapper for our declaration.  This just expands out
   // NS_IMPL_CYCLE_COLLECTION_TRACE_PRESERVED_WRAPPER which we can't use
@@ -361,20 +358,17 @@ nsCSSFontFaceRule::IsCCLeaf() const
 {
   if (!Rule::IsCCLeaf()) {
     return false;
   }
 
   return !mDecl.PreservingWrapper();
 }
 
-// QueryInterface implementation for nsCSSFontFaceRule
-NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsCSSFontFaceRule)
-  NS_INTERFACE_MAP_ENTRY(nsIDOMCSSFontFaceRule)
-NS_INTERFACE_MAP_END_INHERITING(Rule)
+NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED_0(nsCSSFontFaceRule, mozilla::css::Rule)
 
 #ifdef DEBUG
 void
 nsCSSFontFaceRule::List(FILE* out, int32_t aIndent) const
 {
   nsCString baseInd, descInd;
   for (int32_t indent = aIndent; --indent >= 0; ) {
     baseInd.AppendLiteral("  ");
@@ -425,23 +419,16 @@ nsCSSFontFaceRule::GetCssTextImpl(nsAStr
 }
 
 nsICSSDeclaration*
 nsCSSFontFaceRule::Style()
 {
   return &mDecl;
 }
 
-NS_IMETHODIMP
-nsCSSFontFaceRule::GetStyle(nsIDOMCSSStyleDeclaration** aStyle)
-{
-  NS_IF_ADDREF(*aStyle = &mDecl);
-  return NS_OK;
-}
-
 // Arguably these should forward to nsCSSFontFaceStyleDecl methods.
 void
 nsCSSFontFaceRule::SetDesc(nsCSSFontDesc aDescID, nsCSSValue const & aValue)
 {
   NS_PRECONDITION(aDescID > eCSSFontDesc_UNKNOWN &&
                   aDescID < eCSSFontDesc_COUNT,
                   "aDescID out of range in nsCSSFontFaceRule::SetDesc");
 

--- a/layout/style/nsCSSFontFaceRule.h
+++ b/layout/style/nsCSSFontFaceRule.h
@@ -5,17 +5,16 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #ifndef nsCSSFontFaceRule_h
 #define nsCSSFontFaceRule_h
 
 #include "mozilla/css/Rule.h"
 #include "nsCSSValue.h"
 #include "nsICSSDeclaration.h"
-#include "nsIDOMCSSFontFaceRule.h"
 
 namespace mozilla {
 
 namespace dom {
 class DocGroup;
 } // namespace dom
 
 struct CSSFontFaceDescriptors
@@ -70,18 +69,17 @@ protected:
 
 private:
   // NOT TO BE IMPLEMENTED
   // This object cannot be allocated on its own, only as part of
   // nsCSSFontFaceRule.
   void* operator new(size_t size) CPP_THROW_NEW;
 };
 
-class nsCSSFontFaceRule final : public mozilla::css::Rule,
-                                public nsIDOMCSSFontFaceRule
+class nsCSSFontFaceRule final : public mozilla::css::Rule
 {
 public:
   nsCSSFontFaceRule(uint32_t aLineNumber, uint32_t aColumnNumber)
     : mozilla::css::Rule(aLineNumber, aColumnNumber)
   {
   }
 
   nsCSSFontFaceRule(const nsCSSFontFaceRule& aCopy)
@@ -97,19 +95,16 @@ public:
 
 #ifdef DEBUG
   virtual void List(FILE* out = stdout, int32_t aIndent = 0) const override;
 #endif
   virtual int32_t GetType() const override;
   using Rule::GetType;
   virtual already_AddRefed<mozilla::css::Rule> Clone() const override;
 
-  // nsIDOMCSSFontFaceRule interface
-  NS_DECL_NSIDOMCSSFONTFACERULE
-
   void SetDesc(nsCSSFontDesc aDescID, nsCSSValue const & aValue);
   void GetDesc(nsCSSFontDesc aDescID, nsCSSValue & aValue);
 
   // WebIDL interface
   uint16_t Type() const override;
   void GetCssTextImpl(nsAString& aCssText) const override;
   nsICSSDeclaration* Style();