diff --git a/dom/bindings/DOMJSProxyHandler.cpp b/dom/bindings/DOMJSProxyHandler.cpp
index 532e9015cab6..4cb4dd1c1e76 100644
--- a/dom/bindings/DOMJSProxyHandler.cpp
+++ b/dom/bindings/DOMJSProxyHandler.cpp
@@ -219,6 +219,45 @@ bool DOMProxyHandler::defineProperty(JSContext* cx, JS::Handle<JSObject*> proxy,
   return true;
 }
 
+bool DOMProxyHandler::definePrivateField(JSContext* cx, HandleObject proxy,
+                                         HandleId id,
+                                         Handle<PropertyDescriptor> desc,
+                                         ObjectOpResult& result) const {
+  // Delegate to defineProperty, since InitPrivateElemOperation will
+  // do the pre-existence check for us.
+  return this->defineProperty(cx, proxy, id, desc, result);
+}
+
+bool DOMProxyHandler::setPrivate(JSContext* cx, Handle<JSObject*> proxy,
+                                 Handle<jsid> id, Handle<JS::Value> v,
+                                 Handle<JS::Value> receiver,
+                                 ObjectOpResult& result) const {
+  // Delegate to set, since SetPrivateElemOperation will
+  // do the pre-existence check for us.
+  return this->set(cx, proxy, id, v, receiver, result);
+}
+
+bool DOMProxyHandler::getPrivate(JSContext* cx, HandleObject proxy,
+                                 HandleValue receiver, HandleId id,
+                                 MutableHandleValue vp) const {
+  // Delegate to set, since GetPrivateElemOperation will
+  // do the pre-existence check for us.
+  return this->get(cx, proxy, receiver, id, vp);
+}
+
+bool DOMProxyHandler::hasPrivate(JSContext* cx, HandleObject proxy, HandleId id,
+                                 bool* bp) const {
+  JS::Rooted<JSObject*> expando(cx, GetExpandoObject(proxy));
+  // If there is no expando object, then there is no private field.
+  if (!expando) {
+    *bp = false;
+    return true;
+  }
+
+  // Check if the private property is on the expando.
+  return JS_HasOwnPropertyById(cx, expando, id, bp);
+}
+
 bool DOMProxyHandler::set(JSContext* cx, Handle<JSObject*> proxy,
                           Handle<jsid> id, Handle<JS::Value> v,
                           Handle<JS::Value> receiver,
diff --git a/dom/bindings/DOMJSProxyHandler.h b/dom/bindings/DOMJSProxyHandler.h
index 3c670b5cfb03..cbeca8d68b2e 100644
--- a/dom/bindings/DOMJSProxyHandler.h
+++ b/dom/bindings/DOMJSProxyHandler.h
@@ -113,6 +113,23 @@ class DOMProxyHandler : public BaseDOMProxyHandler {
            JS::Handle<JS::Value> v, JS::Handle<JS::Value> receiver,
            JS::ObjectOpResult& result) const override;
 
+  // Override the Private Fields code to instead use the DOM Expando object
+  // rather than the Proxy Expando object.
+  virtual bool hasPrivate(JSContext* cx, JS::Handle<JSObject*> proxy,
+                          JS::Handle<jsid> id, bool* bp) const override;
+  virtual bool getPrivate(JSContext* cx, JS::Handle<JSObject*> proxy,
+                          JS::Handle<JS::Value> receiver, JS::Handle<jsid> id,
+                          JS::MutableHandle<JS::Value> vp) const override;
+  virtual bool setPrivate(JSContext* cx, JS::Handle<JSObject*> proxy,
+                          JS::Handle<jsid> id, JS::Handle<JS::Value> v,
+                          JS::Handle<JS::Value> receiver,
+                          JS::ObjectOpResult& result) const override;
+
+  virtual bool definePrivateField(JSContext* cx, JS::Handle<JSObject*> proxy,
+                                  JS::Handle<jsid> id,
+                                  JS::Handle<JS::PropertyDescriptor> desc,
+                                  JS::ObjectOpResult& result) const override;
+
   /*
    * If assigning to proxy[id] hits a named setter with OverrideBuiltins or
    * an indexed setter, call it and set *done to true on success. Otherwise, set