--- a/layout/style/CounterStyleManager.cpp
+++ b/layout/style/CounterStyleManager.cpp
@@ -984,24 +984,25 @@ public:
   {
     return aPresContext->PresShell()->AllocateByObjectID(
         eArenaObjectID_DependentBuiltinCounterStyle, sz);
   }
 
 private:
   void Destroy()
   {
+    MOZ_ASSERT(NS_IsMainThread());
     nsIPresShell* shell = mManager->PresContext()->PresShell();
     this->~DependentBuiltinCounterStyle();
     shell->FreeByObjectID(eArenaObjectID_DependentBuiltinCounterStyle, this);
   }
 
   CounterStyleManager* mManager;
 
-  nsAutoRefCnt mRefCnt;
+  ThreadSafeAutoRefCnt mRefCnt;
   NS_DECL_OWNINGTHREAD
 };
 
 NS_IMPL_ADDREF(DependentBuiltinCounterStyle)
 NS_IMPL_RELEASE_WITH_DESTROY(DependentBuiltinCounterStyle, Destroy())
 
 /* virtual */ CounterStyle*
 DependentBuiltinCounterStyle::GetFallback()
@@ -1105,16 +1106,17 @@ public:
   {
     return aPresContext->PresShell()->AllocateByObjectID(
         eArenaObjectID_CustomCounterStyle, sz);
   }
 
 private:
   void Destroy()
   {
+    MOZ_ASSERT(NS_IsMainThread());
     nsIPresShell* shell = mManager->PresContext()->PresShell();
     this->~CustomCounterStyle();
     shell->FreeByObjectID(eArenaObjectID_CustomCounterStyle, this);
   }
 
   const nsTArray<nsString>& GetSymbols();
   const nsTArray<AdditiveSymbol>& GetAdditiveSymbols();
 
@@ -1186,17 +1188,17 @@ private:
   CounterStyle* mSpeakAsCounter;
 
   CounterStyle* mExtends;
   // This field refers to the last counter in the extends chain. The
   // counter must be either a builtin style or a style whose system is
   // not 'extends'.
   CounterStyle* mExtendsRoot;
 
-  nsAutoRefCnt mRefCnt;
+  ThreadSafeAutoRefCnt mRefCnt;
   NS_DECL_OWNINGTHREAD
 };
 
 NS_IMPL_ADDREF(CustomCounterStyle)
 NS_IMPL_RELEASE_WITH_DESTROY(CustomCounterStyle, Destroy())
 
 void
 CustomCounterStyle::ResetCachedData()

--- a/layout/style/CounterStyleManager.h
+++ b/layout/style/CounterStyleManager.h
@@ -127,17 +127,17 @@ public:
                                      bool& aIsRTL) override;
 
   virtual AnonymousCounterStyle* AsAnonymous() override { return this; }
 
   bool IsSingleString() const { return mSingleString; }
   uint8_t GetSystem() const { return mSystem; }
   const nsTArray<nsString>& GetSymbols() const { return mSymbols; }
 
-  NS_INLINE_DECL_REFCOUNTING(AnonymousCounterStyle, override)
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AnonymousCounterStyle, override)
 
 private:
   ~AnonymousCounterStyle() {}
 
   bool mSingleString;
   uint8_t mSystem;
   nsTArray<nsString> mSymbols;
 };