--- a/dom/base/nsIObjectLoadingContent.idl
+++ b/dom/base/nsIObjectLoadingContent.idl
@@ -20,17 +20,17 @@ class nsNPAPIPluginInstance;
 
 /**
  * This interface represents a content node that loads objects.
  *
  * Please make sure to update the MozObjectLoadingContent WebIDL
  * interface to mirror this interface when changing it.
  */
 
-[scriptable, uuid(16c14177-52eb-49d3-9842-a1a0b92be11a)]
+[scriptable, uuid(5efbd411-5bbe-4de1-9f3a-1c3459696eb2)]
 interface nsIObjectLoadingContent : nsISupports
 {
   /**
    * See notes in nsObjectLoadingContent.h
    */
   const unsigned long TYPE_LOADING  = 0;
   const unsigned long TYPE_IMAGE    = 1;
   const unsigned long TYPE_PLUGIN   = 2;
@@ -183,9 +183,17 @@ interface nsIObjectLoadingContent : nsIS
    * not one is pending spawn/despawn.
    */
   readonly attribute bool hasRunningPlugin;
 
   /**
    * This method will disable the play-preview plugin state.
    */
   void cancelPlayPreview();
+
+  /**
+   * If this plugin runs out-of-process, it has a runID to differentiate
+   * between different times the plugin process has been instantiated.
+   *
+   * This throws NS_ERROR_NOT_IMPLEMENTED for in-process plugins.
+   */
+  readonly attribute unsigned long runID;
 };

--- a/dom/base/nsObjectLoadingContent.cpp
+++ b/dom/base/nsObjectLoadingContent.cpp
@@ -696,16 +696,18 @@ nsObjectLoadingContent::UnbindFromTree(b
                                                        NS_LITERAL_STRING("PluginRemoved"));
     NS_DispatchToCurrentThread(ev);
   }
 }
 
 nsObjectLoadingContent::nsObjectLoadingContent()
   : mType(eType_Loading)
   , mFallbackType(eFallbackAlternate)
+  , mRunID(0)
+  , mHasRunID(false)
   , mChannelLoaded(false)
   , mInstantiating(false)
   , mNetworkCreated(true)
   , mActivated(false)
   , mPlayPreviewCanceled(false)
   , mIsStopping(false)
   , mIsLoading(false)
   , mScriptRequested(false) {}
@@ -810,16 +812,27 @@ nsObjectLoadingContent::InstantiatePlugi
       }
       newOwner->Destroy();
     }
     return NS_OK;
   }
 
   mInstanceOwner = newOwner;
 
+  if (mInstanceOwner) {
+    nsRefPtr<nsNPAPIPluginInstance> inst;
+    rv = mInstanceOwner->GetInstance(getter_AddRefs(inst));
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+
+    rv = inst->GetRunID(&mRunID);
+    mHasRunID = NS_SUCCEEDED(rv);
+  }
+
   // Ensure the frame did not change during instantiation re-entry (common).
   // HasNewFrame would not have mInstanceOwner yet, so the new frame would be
   // dangling. (Bug 854082)
   nsIFrame* frame = thisContent->GetPrimaryFrame();
   if (frame && mInstanceOwner) {
     mInstanceOwner->SetFrame(static_cast<nsPluginFrame*>(frame));
 
     // Bug 870216 - Adobe Reader renders with incorrect dimensions until it gets
@@ -3140,16 +3153,34 @@ nsObjectLoadingContent::CancelPlayPrevie
   // If we're in play preview state already, reload
   if (mType == eType_Null && mFallbackType == eFallbackPlayPreview) {
     return LoadObject(true, true);
   }
 
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsObjectLoadingContent::GetRunID(uint32_t* aRunID)
+{
+  if (NS_WARN_IF(!nsContentUtils::IsCallerChrome())) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+  if (NS_WARN_IF(!aRunID)) {
+    return NS_ERROR_INVALID_POINTER;
+  }
+  if (!mHasRunID) {
+    // The plugin instance must not have a run ID, so we must
+    // be running the plugin in-process.
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  *aRunID = mRunID;
+  return NS_OK;
+}
+
 static bool sPrefsInitialized;
 static uint32_t sSessionTimeoutMinutes;
 static uint32_t sPersistentTimeoutDays;
 
 bool
 nsObjectLoadingContent::ShouldPlay(FallbackType &aReason, bool aIgnoreCurrentType)
 {
   nsresult rv;

--- a/dom/base/nsObjectLoadingContent.h
+++ b/dom/base/nsObjectLoadingContent.h
@@ -228,16 +228,28 @@ class nsObjectLoadingContent : public ns
     {
       aRv.Throw(NS_ERROR_NOT_IMPLEMENTED);
     }
     void LegacyCall(JSContext* aCx, JS::Handle<JS::Value> aThisVal,
                     const mozilla::dom::Sequence<JS::Value>& aArguments,
                     JS::MutableHandle<JS::Value> aRetval,
                     mozilla::ErrorResult& aRv);
 
+    uint32_t GetRunID(mozilla::ErrorResult& aRv)
+    {
+      uint32_t runID;
+      nsresult rv = GetRunID(&runID);
+      if (NS_FAILED(rv)) {
+        aRv.Throw(rv);
+        return 0;
+      }
+
+      return runID;
+    }
+
   protected:
     /**
      * Begins loading the object when called
      *
      * Attributes of |this| QI'd to nsIContent will be inspected, depending on
      * the node type. This function currently assumes it is a <applet>,
      * <object>, or <embed> tag.
      *
@@ -574,16 +586,19 @@ class nsObjectLoadingContent : public ns
 
 
 
     // Type of the currently-loaded content.
     ObjectType                  mType           : 8;
     // The type of fallback content we're showing (see ObjectState())
     FallbackType                mFallbackType : 8;
 
+    uint32_t                    mRunID;
+    bool                        mHasRunID;
+
     // If true, we have opened a channel as the listener and it has reached
     // OnStartRequest. Does not get set for channels that are passed directly to
     // the plugin listener.
     bool                        mChannelLoaded    : 1;
 
     // Whether we are about to call instantiate on our frame. If we aren't,
     // SetFrame needs to asynchronously call Instantiate.
     bool                        mInstantiating : 1;

--- a/dom/plugins/base/PluginPRLibrary.h
+++ b/dom/plugins/base/PluginPRLibrary.h
@@ -117,16 +117,17 @@ public:
     virtual nsresult ContentsScaleFactorChanged(NPP aInstance, double aContentsScaleFactor) override;
 #endif
     virtual nsresult SetBackgroundUnknown(NPP instance) override;
     virtual nsresult BeginUpdateBackground(NPP instance,
                                            const nsIntRect&, gfxContext** aCtx) override;
     virtual nsresult EndUpdateBackground(NPP instance,
                                          gfxContext* aCtx, const nsIntRect&) override;
     virtual void GetLibraryPath(nsACString& aPath) { aPath.Assign(mFilePath); }
+    virtual nsresult GetRunID(uint32_t* aRunID) override { return NS_ERROR_NOT_IMPLEMENTED; }
 
 private:
     NP_InitializeFunc mNP_Initialize;
     NP_ShutdownFunc mNP_Shutdown;
     NP_GetMIMEDescriptionFunc mNP_GetMIMEDescription;
 #if defined(XP_UNIX) && !defined(XP_MACOSX)
     NP_GetValueFunc mNP_GetValue;
 #endif

--- a/dom/plugins/base/nsNPAPIPluginInstance.cpp
+++ b/dom/plugins/base/nsNPAPIPluginInstance.cpp
@@ -1773,8 +1773,27 @@ double
 nsNPAPIPluginInstance::GetContentsScaleFactor()
 {
   double scaleFactor = 1.0;
   if (mOwner) {
     mOwner->GetContentsScaleFactor(&scaleFactor);
   }
   return scaleFactor;
 }
+
+nsresult
+nsNPAPIPluginInstance::GetRunID(uint32_t* aRunID)
+{
+  if (NS_WARN_IF(!aRunID)) {
+    return NS_ERROR_INVALID_POINTER;
+  }
+
+  if (NS_WARN_IF(!mPlugin)) {
+    return NS_ERROR_FAILURE;
+  }
+
+  PluginLibrary* library = mPlugin->GetLibrary();
+  if (!library) {
+    return NS_ERROR_FAILURE;
+  }
+
+  return library->GetRunID(aRunID);
+}

--- a/dom/plugins/base/nsNPAPIPluginInstance.h
+++ b/dom/plugins/base/nsNPAPIPluginInstance.h
@@ -281,16 +281,18 @@ public:
 
   // Called when the instance fails to instantiate beceause the Carbon
   // event model is not supported.
   void CarbonNPAPIFailure();
 
   // Returns the contents scale factor of the screen the plugin is drawn on.
   double GetContentsScaleFactor();
 
+  nsresult GetRunID(uint32_t *aRunID);
+
   static bool InPluginCallUnsafeForReentry() { return gInUnsafePluginCalls > 0; }
   static void BeginPluginCall(NSPluginCallReentry aReentryState)
   {
     if (aReentryState == NS_PLUGIN_CALL_UNSAFE_TO_REENTER_GECKO) {
       ++gInUnsafePluginCalls;
     }
   }
   static void EndPluginCall(NSPluginCallReentry aReentryState)

--- a/dom/plugins/ipc/PluginLibrary.h
+++ b/dom/plugins/ipc/PluginLibrary.h
@@ -79,14 +79,15 @@ public:
    * PluginInstanceParent.  They approximately follow the ReadbackSink
    * API.
    */
   virtual nsresult SetBackgroundUnknown(NPP instance) = 0;
   virtual nsresult BeginUpdateBackground(NPP instance,
                                          const nsIntRect&, gfxContext**) = 0;
   virtual nsresult EndUpdateBackground(NPP instance,
                                        gfxContext*, const nsIntRect&) = 0;
+  virtual nsresult GetRunID(uint32_t* aRunID) = 0;
 };
 
 
 } // namespace mozilla
 
 #endif  // ifndef mozilla_PluginLibrary_h

--- a/dom/plugins/ipc/PluginModuleParent.h
+++ b/dom/plugins/ipc/PluginModuleParent.h
@@ -123,17 +123,17 @@ public:
     void ProcessRemoteNativeEventsInInterruptCall() override;
 
     virtual bool WaitForIPCConnection() { return true; }
 
     nsCString GetHistogramKey() const {
         return mPluginName + mPluginVersion;
     }
 
-    nsresult GetRunID(uint32_t* aRunID);
+    virtual nsresult GetRunID(uint32_t* aRunID) override;
 
 protected:
     virtual mozilla::ipc::RacyInterruptPolicy
     MediateInterruptRace(const Message& parent, const Message& child) override
     {
         return MediateRace(parent, child);
     }
 

--- a/dom/webidl/HTMLObjectElement.webidl
+++ b/dom/webidl/HTMLObjectElement.webidl
@@ -206,16 +206,19 @@ interface MozObjectLoadingContent {
   [ChromeOnly]
   readonly attribute boolean hasRunningPlugin;
 
   /**
    * This method will disable the play-preview plugin state.
    */
   [ChromeOnly, Throws]
   void cancelPlayPreview();
+
+  [ChromeOnly, Throws]
+  readonly attribute unsigned long runID;
 };
 
 /**
  * Name:Value pair type used for passing parameters to NPAPI or javascript
  * plugins.
  */
 dictionary MozPluginParameter {
   DOMString name = "";