diff --git a/intl/unicharutil/util/nsUnicharUtils.h b/intl/unicharutil/util/nsUnicharUtils.h
index 3f33c851da83..af4b2843c161 100644
--- a/intl/unicharutil/util/nsUnicharUtils.h
+++ b/intl/unicharutil/util/nsUnicharUtils.h
@@ -45,7 +45,7 @@ public:
   virtual int32_t operator() (const char16_t*,
                               const char16_t*,
                               uint32_t,
-                              uint32_t) const;
+                              uint32_t) const MOZ_OVERRIDE;
 };
 
 class nsCaseInsensitiveUTF8StringComparator : public nsCStringComparator
@@ -54,7 +54,7 @@ public:
   virtual int32_t operator() (const char*,
                               const char*,
                               uint32_t,
-                              uint32_t) const;
+                              uint32_t) const MOZ_OVERRIDE;
 };
 
 class nsCaseInsensitiveStringArrayComparator
@@ -73,7 +73,7 @@ public:
   virtual int operator() (const char16_t*,
                           const char16_t*,
                           uint32_t,
-                          uint32_t) const;
+                          uint32_t) const MOZ_OVERRIDE;
 };
 
 inline bool
diff --git a/js/xpconnect/idl/nsIXPCScriptable.idl b/js/xpconnect/idl/nsIXPCScriptable.idl
index e3962d1222fb..0117c1439e5f 100644
--- a/js/xpconnect/idl/nsIXPCScriptable.idl
+++ b/js/xpconnect/idl/nsIXPCScriptable.idl
@@ -156,8 +156,8 @@ class NS_NO_VTABLE nsXPCClassInfo : public nsIClassInfo,
 public:
   NS_DECLARE_STATIC_IID_ACCESSOR(NS_XPCCLASSINFO_IID)
 
-  NS_IMETHOD_(MozExternalRefCountType) AddRef() = 0;
-  NS_IMETHOD_(MozExternalRefCountType) Release() = 0;
+  NS_IMETHOD_(MozExternalRefCountType) AddRef() MOZ_OVERRIDE = 0;
+  NS_IMETHOD_(MozExternalRefCountType) Release() MOZ_OVERRIDE = 0;
 
   virtual void PreserveWrapper(nsISupports *aNative) = 0;
 };
diff --git a/xpcom/base/CycleCollectedJSRuntime.h b/xpcom/base/CycleCollectedJSRuntime.h
index c3845c2d403e..c3bf60b30c41 100644
--- a/xpcom/base/CycleCollectedJSRuntime.h
+++ b/xpcom/base/CycleCollectedJSRuntime.h
@@ -29,27 +29,28 @@ namespace mozilla {
 class JSGCThingParticipant: public nsCycleCollectionParticipant
 {
 public:
-  NS_IMETHOD_(void) Root(void*)
+  NS_IMETHOD_(void) Root(void*) MOZ_OVERRIDE
   {
     MOZ_ASSERT(false, "Don't call Root on GC things");
   }
 
-  NS_IMETHOD_(void) Unlink(void*)
+  NS_IMETHOD_(void) Unlink(void*) MOZ_OVERRIDE
   {
     MOZ_ASSERT(false, "Don't call Unlink on GC things, as they may be dead");
   }
 
-  NS_IMETHOD_(void) Unroot(void*)
+  NS_IMETHOD_(void) Unroot(void*) MOZ_OVERRIDE
   {
     MOZ_ASSERT(false, "Don't call Unroot on GC things, as they may be dead");
   }
 
-  NS_IMETHOD_(void) DeleteCycleCollectable(void* aPtr)
+  NS_IMETHOD_(void) DeleteCycleCollectable(void* aPtr) MOZ_OVERRIDE
   {
     MOZ_ASSERT(false, "Can't directly delete a cycle collectable GC thing");
   }
 
-  NS_IMETHOD Traverse(void* aPtr, nsCycleCollectionTraversalCallback& aCb);
+  NS_IMETHOD Traverse(void* aPtr, nsCycleCollectionTraversalCallback& aCb)
+    MOZ_OVERRIDE;
 };
 
 class JSZoneParticipant : public nsCycleCollectionParticipant
@@ -59,27 +60,28 @@ public:
   {
   }
 
-  NS_IMETHOD_(void) Root(void*)
+  NS_IMETHOD_(void) Root(void*) MOZ_OVERRIDE
   {
     MOZ_ASSERT(false, "Don't call Root on GC things");
   }
 
-  NS_IMETHOD_(void) Unlink(void*)
+  NS_IMETHOD_(void) Unlink(void*) MOZ_OVERRIDE
   {
     MOZ_ASSERT(false, "Don't call Unlink on GC things, as they may be dead");
   }
 
-  NS_IMETHOD_(void) Unroot(void*)
+  NS_IMETHOD_(void) Unroot(void*) MOZ_OVERRIDE
   {
     MOZ_ASSERT(false, "Don't call Unroot on GC things, as they may be dead");
   }
 
-  NS_IMETHOD_(void) DeleteCycleCollectable(void*)
+  NS_IMETHOD_(void) DeleteCycleCollectable(void*) MOZ_OVERRIDE
   {
     MOZ_ASSERT(false, "Can't directly delete a cycle collectable GC thing");
   }
 
-  NS_IMETHOD Traverse(void* aPtr, nsCycleCollectionTraversalCallback& aCb);
+  NS_IMETHOD Traverse(void* aPtr, nsCycleCollectionTraversalCallback& aCb)
+    MOZ_OVERRIDE;
 };
 
 class IncrementalFinalizeRunnable;
diff --git a/xpcom/base/nsAutoPtr.h b/xpcom/base/nsAutoPtr.h
index ff83f97a0f82..f56503c871b2 100644
--- a/xpcom/base/nsAutoPtr.h
+++ b/xpcom/base/nsAutoPtr.h
@@ -771,7 +771,7 @@ public:
   }
 
   virtual nsresult NS_FASTCALL operator()(const nsIID& aIID,
-                                          void** aResult) const
+                                          void** aResult) const MOZ_OVERRIDE
   {
     nsresult status = mRawPtr ? mRawPtr->QueryInterface(aIID, aResult)
                               : NS_ERROR_NULL_POINTER;
diff --git a/xpcom/ds/nsICollection.idl b/xpcom/ds/nsICollection.idl
index 3e7c1f087d16..afa7ac7d2dac 100644
--- a/xpcom/ds/nsICollection.idl
+++ b/xpcom/ds/nsICollection.idl
@@ -42,7 +42,8 @@ class MOZ_STACK_CLASS nsQueryElementAt : public nsCOMPtr_helper
           // nothing else to do here
         }
 
-      virtual nsresult NS_FASTCALL operator()( const nsIID& aIID, void** ) const;
+      virtual nsresult NS_FASTCALL operator()( const nsIID& aIID, void** )
+        const MOZ_OVERRIDE;
 
     private:
       nsICollection* MOZ_NON_OWNING_REF mCollection;
diff --git a/xpcom/glue/nsArrayUtils.h b/xpcom/glue/nsArrayUtils.h
index d380e9cd9bc8..e216c18c2697 100644
--- a/xpcom/glue/nsArrayUtils.h
+++ b/xpcom/glue/nsArrayUtils.h
@@ -22,7 +22,8 @@ public:
   {
   }
 
-  virtual nsresult NS_FASTCALL operator()(const nsIID& aIID, void**) const;
+  virtual nsresult NS_FASTCALL operator()(const nsIID& aIID, void**) const
+    MOZ_OVERRIDE;
 
 private:
   nsIArray* MOZ_NON_OWNING_REF mArray;
diff --git a/xpcom/glue/nsComponentManagerUtils.h b/xpcom/glue/nsComponentManagerUtils.h
index f8b4da2e952a..aa40f6891c28 100644
--- a/xpcom/glue/nsComponentManagerUtils.h
+++ b/xpcom/glue/nsComponentManagerUtils.h
@@ -37,7 +37,8 @@ public:
   {
   }
 
-  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const;
+  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const
+    MOZ_OVERRIDE;
 
 private:
   const nsCID&    mCID;
@@ -56,7 +57,7 @@ public:
   {
   }
 
-  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const;
+  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const MOZ_OVERRIDE;
 
 private:
   const char*   mContractID;
@@ -75,7 +76,7 @@ public:
   {
   }
 
-  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const;
+  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const MOZ_OVERRIDE;
 
 private:
   nsIFactory* MOZ_NON_OWNING_REF mFactory;
@@ -132,7 +133,7 @@ public:
   {
   }
 
-  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const;
+  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const MOZ_OVERRIDE;
 
 private:
   const nsCID&    mCID;
@@ -148,7 +149,7 @@ public:
   {
   }
 
-  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const;
+  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const MOZ_OVERRIDE;
 
 private:
   const char*   mContractID;
diff --git a/xpcom/glue/nsCycleCollectionParticipant.h b/xpcom/glue/nsCycleCollectionParticipant.h
index 1fcf858f75b8..03c8078e0eec 100644
--- a/xpcom/glue/nsCycleCollectionParticipant.h
+++ b/xpcom/glue/nsCycleCollectionParticipant.h
@@ -182,7 +182,7 @@ public:
   }
 
   NS_IMETHOD_(void) Trace(void* aPtr, const TraceCallbacks& aCb,
-                          void* aClosure) = 0;
+                          void* aClosure) MOZ_OVERRIDE = 0;
 
   static void NoteJSChild(JS::GCCellPtr aGCThing, const char* aName,
                           void* aClosure);
@@ -202,11 +202,11 @@ public:
 
   NS_DECLARE_STATIC_IID_ACCESSOR(NS_XPCOMCYCLECOLLECTIONPARTICIPANT_IID)
 
-  NS_IMETHOD_(void) Root(void* aPtr);
-  NS_IMETHOD_(void) Unroot(void* aPtr);
+  NS_IMETHOD_(void) Root(void* aPtr) MOZ_OVERRIDE;
+  NS_IMETHOD_(void) Unroot(void* aPtr) MOZ_OVERRIDE;
 
   NS_IMETHOD_(void) Trace(void* aPtr, const TraceCallbacks& aCb,
-                          void* aClosure);
+                          void* aClosure) MOZ_OVERRIDE;
 
   static bool CheckForRightISupports(nsISupports* aSupports);
 };
@@ -507,8 +507,9 @@ DowncastCCParticipant(void* aPtr)
 
 #define NS_DECL_CYCLE_COLLECTION_CLASS_BODY_NO_UNLINK(_class, _base)           \
 public:                                                                        \
-  NS_IMETHOD Traverse(void *p, nsCycleCollectionTraversalCallback &cb);        \
-  NS_IMETHOD_(void) DeleteCycleCollectable(void *p)                            \
+  NS_IMETHOD Traverse(void *p, nsCycleCollectionTraversalCallback &cb)         \
+    MOZ_OVERRIDE;                                                              \
+  NS_IMETHOD_(void) DeleteCycleCollectable(void *p) MOZ_OVERRIDE               \
   {                                                                            \
     DowncastCCParticipant<_class>(p)->DeleteCycleCollectable();                \
   }                                                                            \
@@ -526,7 +527,7 @@ public:                                                                        \
 
 #define NS_DECL_CYCLE_COLLECTION_CLASS_BODY(_class, _base)                     \
   NS_DECL_CYCLE_COLLECTION_CLASS_BODY_NO_UNLINK(_class, _base)                 \
-  NS_IMETHOD_(void) Unlink(void *p);
+  NS_IMETHOD_(void) Unlink(void *p) MOZ_OVERRIDE;
 
 #define NS_PARTICIPANT_AS(type, participant)                                   \
   const_cast<type*>(reinterpret_cast<const type*>(participant))
@@ -573,9 +574,9 @@ public:
   : nsXPCOMCycleCollectionParticipant(true) {}                                   \
 private:                                                                         \
   NS_DECL_CYCLE_COLLECTION_CLASS_BODY(_class, _base)                             \
-  NS_IMETHOD_(bool) CanSkipReal(void *p, bool aRemovingAllowed);                 \
-  NS_IMETHOD_(bool) CanSkipInCCReal(void *p);                                    \
-  NS_IMETHOD_(bool) CanSkipThisReal(void *p);                                    \
+  NS_IMETHOD_(bool) CanSkipReal(void *p, bool aRemovingAllowed) MOZ_OVERRIDE;    \
+  NS_IMETHOD_(bool) CanSkipInCCReal(void *p) MOZ_OVERRIDE;                       \
+  NS_IMETHOD_(bool) CanSkipThisReal(void *p) MOZ_OVERRIDE;                       \
   NS_IMPL_GET_XPCOM_CYCLE_COLLECTION_PARTICIPANT(_class)                         \
 };                                                                               \
 NS_CHECK_FOR_RIGHT_PARTICIPANT_IMPL(_class)                                      \
@@ -590,7 +591,8 @@ class NS_CYCLE_COLLECTION_INNERCLASS
  : public nsXPCOMCycleCollectionParticipant                                            \
 {                                                                                      \
   NS_DECL_CYCLE_COLLECTION_CLASS_BODY(_class, _base)                                   \
-  NS_IMETHOD_(void) Trace(void *p, const TraceCallbacks &cb, void *closure);           \
+  NS_IMETHOD_(void) Trace(void *p, const TraceCallbacks &cb, void *closure)            \
+    MOZ_OVERRIDE;                                                                      \
   NS_IMPL_GET_XPCOM_CYCLE_COLLECTION_PARTICIPANT(_class)                               \
 };                                                                                     \
 NS_CHECK_FOR_RIGHT_PARTICIPANT_IMPL(_class)                                            \
@@ -606,10 +608,10 @@ public:
   : nsXPCOMCycleCollectionParticipant(true) {}                                            \
 private:                                                                                  \
   NS_DECL_CYCLE_COLLECTION_CLASS_BODY(_class, _base)                                      \
-  NS_IMETHOD_(void) Trace(void *p, const TraceCallbacks &cb, void *closure);              \
-  NS_IMETHOD_(bool) CanSkipReal(void *p, bool aRemovingAllowed);                          \
-  NS_IMETHOD_(bool) CanSkipInCCReal(void *p);                                             \
-  NS_IMETHOD_(bool) CanSkipThisReal(void *p);                                             \
+  NS_IMETHOD_(void) Trace(void *p, const TraceCallbacks &cb, void *closure) MOZ_OVERRIDE; \
+  NS_IMETHOD_(bool) CanSkipReal(void *p, bool aRemovingAllowed) MOZ_OVERRIDE;             \
+  NS_IMETHOD_(bool) CanSkipInCCReal(void *p) MOZ_OVERRIDE;                                \
+  NS_IMETHOD_(bool) CanSkipThisReal(void *p) MOZ_OVERRIDE;                                \
   NS_IMPL_GET_XPCOM_CYCLE_COLLECTION_PARTICIPANT(_class)                                  \
 };                                                                                        \
 NS_CHECK_FOR_RIGHT_PARTICIPANT_IMPL(_class)                                               \
@@ -640,7 +642,8 @@ static NS_CYCLE_COLLECTION_INNERCLASS NS_CYCLE_COLLECTION_INNERNAME;
 #define NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED_BODY_NO_UNLINK(_class,        \
                                                                 _base_class)   \
 public:                                                                        \
-  NS_IMETHOD Traverse(void *p, nsCycleCollectionTraversalCallback &cb);        \
+  NS_IMETHOD Traverse(void *p, nsCycleCollectionTraversalCallback &cb)         \
+    MOZ_OVERRIDE;                                                              \
   static _class* Downcast(nsISupports* s)                                      \
   {                                                                            \
     return static_cast<_class*>(static_cast<_base_class*>(                     \
@@ -649,7 +652,7 @@ public:                                                                        \
 
 #define NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED_BODY(_class, _base_class)     \
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED_BODY_NO_UNLINK(_class, _base_class) \
-  NS_IMETHOD_(void) Unlink(void *p);
+  NS_IMETHOD_(void) Unlink(void *p) MOZ_OVERRIDE;
 
 #define NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(_class, _base_class)          \
 class NS_CYCLE_COLLECTION_INNERCLASS                                           \
@@ -680,7 +683,8 @@ class NS_CYCLE_COLLECTION_INNERCLASS
  : public NS_CYCLE_COLLECTION_CLASSNAME(_base_class)                                   \
 {                                                                                      \
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED_BODY(_class, _base_class)                   \
-  NS_IMETHOD_(void) Trace(void *p, const TraceCallbacks &cb, void *closure);           \
+  NS_IMETHOD_(void) Trace(void *p, const TraceCallbacks &cb, void *closure)            \
+    MOZ_OVERRIDE;                                                                      \
   NS_IMPL_GET_XPCOM_CYCLE_COLLECTION_PARTICIPANT(_class)                               \
 };                                                                                     \
 NS_CHECK_FOR_RIGHT_PARTICIPANT_IMPL_INHERITED(_class)                                  \
@@ -690,11 +694,12 @@ static NS_CYCLE_COLLECTION_INNERCLASS NS_CYCLE_COLLECTION_INNERNAME;
 
 #define NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS_BODY(_class)                     \
   public:                                                                      \
-    NS_IMETHOD_(void) Root(void *n);                                           \
-    NS_IMETHOD_(void) Unlink(void *n);                                         \
-    NS_IMETHOD_(void) Unroot(void *n);                                         \
-    NS_IMETHOD Traverse(void *n, nsCycleCollectionTraversalCallback &cb);      \
-    NS_IMETHOD_(void) DeleteCycleCollectable(void *n)                          \
+    NS_IMETHOD_(void) Root(void *n) MOZ_OVERRIDE;                              \
+    NS_IMETHOD_(void) Unlink(void *n) MOZ_OVERRIDE;                            \
+    NS_IMETHOD_(void) Unroot(void *n) MOZ_OVERRIDE;                            \
+    NS_IMETHOD Traverse(void *n, nsCycleCollectionTraversalCallback &cb)       \
+  MOZ_OVERRIDE;                                                                \
+    NS_IMETHOD_(void) DeleteCycleCollectable(void *n) MOZ_OVERRIDE             \
     {                                                                          \
       DowncastCCParticipant<_class>(n)->DeleteCycleCollectable();              \
     }                                                                          \
@@ -736,9 +741,9 @@ static NS_CYCLE_COLLECTION_INNERCLASS NS_CYCLE_COLLECTION_INNERNAME;
     : nsCycleCollectionParticipant(true) {}                                    \
   private:                                                                     \
     NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS_BODY(_class)                         \
-    NS_IMETHOD_(bool) CanSkipReal(void *p, bool aRemovingAllowed);             \
-    NS_IMETHOD_(bool) CanSkipInCCReal(void *p);                                \
-    NS_IMETHOD_(bool) CanSkipThisReal(void *p);                                \
+    NS_IMETHOD_(bool) CanSkipReal(void *p, bool aRemovingAllowed) MOZ_OVERRIDE;\
+    NS_IMETHOD_(bool) CanSkipInCCReal(void *p) MOZ_OVERRIDE;                   \
+    NS_IMETHOD_(bool) CanSkipThisReal(void *p) MOZ_OVERRIDE;                   \
     static nsCycleCollectionParticipant* GetParticipant()                      \
     {                                                                          \
       return &_class::NS_CYCLE_COLLECTION_INNERNAME;                           \
@@ -755,9 +760,9 @@ public:                                                                        \
   : nsCycleCollectionParticipant(true) {}                                      \
 private:                                                                       \
   NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS_BODY(_class)                           \
-  NS_IMETHOD_(bool) CanSkipReal(void *p, bool aRemovingAllowed);               \
-  NS_IMETHOD_(bool) CanSkipInCCReal(void *p);                                  \
-  NS_IMETHOD_(bool) CanSkipThisReal(void *p);                                  \
+  NS_IMETHOD_(bool) CanSkipReal(void *p, bool aRemovingAllowed) MOZ_OVERRIDE;  \
+  NS_IMETHOD_(bool) CanSkipInCCReal(void *p) MOZ_OVERRIDE;                     \
+  NS_IMETHOD_(bool) CanSkipThisReal(void *p) MOZ_OVERRIDE;                     \
   static nsCycleCollectionParticipant* GetParticipant()                        \
   {                                                                            \
     return &_class::NS_CYCLE_COLLECTION_INNERNAME;                             \
@@ -774,7 +779,8 @@ static NS_CYCLE_COLLECTION_INNERCLASS NS_CYCLE_COLLECTION_INNERNAME;
    : public nsScriptObjectTracer                                               \
   {                                                                            \
     NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS_BODY(_class)                         \
-    NS_IMETHOD_(void) Trace(void *p, const TraceCallbacks &cb, void *closure); \
+    NS_IMETHOD_(void) Trace(void *p, const TraceCallbacks &cb, void *closure)  \
+      MOZ_OVERRIDE;                                                            \
     static MOZ_CONSTEXPR nsScriptObjectTracer* GetParticipant()                \
     {                                                                          \
       return &_class::NS_CYCLE_COLLECTION_INNERNAME;                           \
diff --git a/xpcom/glue/nsIWeakReferenceUtils.h b/xpcom/glue/nsIWeakReferenceUtils.h
index 466f5dc59970..2964cb91f781 100644
--- a/xpcom/glue/nsIWeakReferenceUtils.h
+++ b/xpcom/glue/nsIWeakReferenceUtils.h
@@ -39,7 +39,8 @@ public:
   {
   }
 
-  virtual nsresult NS_FASTCALL operator()(const nsIID& aIID, void**) const;
+  virtual nsresult NS_FASTCALL operator()(const nsIID& aIID, void**) const
+    MOZ_OVERRIDE;
 
 private:
   nsIWeakReference* MOZ_NON_OWNING_REF mWeakPtr;
diff --git a/xpcom/glue/nsServiceManagerUtils.h b/xpcom/glue/nsServiceManagerUtils.h
index 026b0929f812..86d5e7da0aa4 100644
--- a/xpcom/glue/nsServiceManagerUtils.h
+++ b/xpcom/glue/nsServiceManagerUtils.h
@@ -45,7 +45,8 @@ public:
   {
   }
 
-  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const;
+  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const
+    MOZ_OVERRIDE;
 protected:
   const char*                 mCategory;
   const char*                 mEntry;
diff --git a/xpcom/string/nsAString.h b/xpcom/string/nsAString.h
index e15ae81aec96..5ad82735d3f6 100644
--- a/xpcom/string/nsAString.h
+++ b/xpcom/string/nsAString.h
@@ -43,7 +43,7 @@ public:
   typedef char char_type;
 
   virtual int operator()(const char_type*, const char_type*,
-                         uint32_t, uint32_t) const;
+                         uint32_t, uint32_t) const MOZ_OVERRIDE;
 };
 
 class nsCaseInsensitiveCStringArrayComparator
diff --git a/xpcom/string/nsTSubstring.h b/xpcom/string/nsTSubstring.h
index a7d5734c1984..313bc9523eef 100644
--- a/xpcom/string/nsTSubstring.h
+++ b/xpcom/string/nsTSubstring.h
@@ -43,7 +43,7 @@ public:
   }
 
   virtual int operator()(const char_type*, const char_type*,
-                         uint32_t, uint32_t) const;
+                         uint32_t, uint32_t) const MOZ_OVERRIDE;
 };
 
 /**