diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index efc59546f841..31e39a750a0f 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -3764,6 +3764,28 @@ nsPIDOMWindowOuter::RefreshMediaElements()
   }
 }
 
+void
+nsPIDOMWindowOuter::SetServiceWorkersTestingEnabled(bool aEnabled)
+{
+  // Devtools should only be setting this on the top level window.  Its
+  // ok if devtools clears the flag on clean up of nested windows, though.
+  // It will have no affect.
+#ifdef DEBUG
+  nsCOMPtr<nsPIDOMWindowOuter> topWindow = GetScriptableTop();
+  MOZ_ASSERT_IF(aEnabled, this == topWindow);
+#endif
+  mServiceWorkersTestingEnabled = aEnabled;
+}
+
+bool
+nsPIDOMWindowOuter::GetServiceWorkersTestingEnabled()
+{
+  // Automatically get this setting from the top level window so that nested
+  // iframes get the correct devtools setting.
+  nsCOMPtr<nsPIDOMWindowOuter> topWindow = GetScriptableTop();
+  return topWindow->mServiceWorkersTestingEnabled;
+}
+
 bool
 nsPIDOMWindowInner::GetAudioCaptured() const
 {
diff --git a/dom/base/nsPIDOMWindow.h b/dom/base/nsPIDOMWindow.h
index 40d28b544ab4..8e356be4aa75 100644
--- a/dom/base/nsPIDOMWindow.h
+++ b/dom/base/nsPIDOMWindow.h
@@ -865,15 +865,8 @@ public:
   float GetAudioVolume() const;
   nsresult SetAudioVolume(float aVolume);
 
-  void SetServiceWorkersTestingEnabled(bool aEnabled)
-  {
-    mServiceWorkersTestingEnabled = aEnabled;
-  }
-
-  bool GetServiceWorkersTestingEnabled()
-  {
-    return mServiceWorkersTestingEnabled;
-  }
+  void SetServiceWorkersTestingEnabled(bool aEnabled);
+  bool GetServiceWorkersTestingEnabled();
 };
 
 NS_DEFINE_STATIC_IID_ACCESSOR(nsPIDOMWindowOuter, NS_PIDOMWINDOWOUTER_IID)