--- a/dom/ipc/TabParent.cpp
+++ b/dom/ipc/TabParent.cpp
@@ -2143,20 +2143,20 @@ mozilla::ipc::IPCResult
 TabParent::RecvStartPluginIME(const WidgetKeyboardEvent& aKeyboardEvent,
                               const int32_t& aPanelX, const int32_t& aPanelY,
                               nsString* aCommitted)
 {
   nsCOMPtr<nsIWidget> widget = GetWidget();
   if (!widget) {
     return IPC_OK();
   }
-  widget->StartPluginIME(aKeyboardEvent,
-                         (int32_t&)aPanelX,
-                         (int32_t&)aPanelY,
-                         *aCommitted);
+  Unused << widget->StartPluginIME(aKeyboardEvent,
+                                   (int32_t&)aPanelX,
+                                   (int32_t&)aPanelY,
+                                   *aCommitted);
   return IPC_OK();
 }
 
 mozilla::ipc::IPCResult
 TabParent::RecvSetPluginFocused(const bool& aFocused)
 {
   nsCOMPtr<nsIWidget> widget = GetWidget();
   if (!widget) {

--- a/widget/PuppetWidget.cpp
+++ b/widget/PuppetWidget.cpp
@@ -676,17 +676,17 @@ PuppetWidget::NotifyIMEInternal(const IM
       return NotifyIMEOfMouseButtonEvent(aIMENotification);
     case NOTIFY_IME_OF_POSITION_CHANGE:
       return NotifyIMEOfPositionChange(aIMENotification);
     default:
       return NS_ERROR_NOT_IMPLEMENTED;
   }
 }
 
-NS_IMETHODIMP
+nsresult
 PuppetWidget::StartPluginIME(const mozilla::WidgetKeyboardEvent& aKeyboardEvent,
                              int32_t aPanelX, int32_t aPanelY,
                              nsString& aCommitted)
 {
   if (!mTabChild ||
       !mTabChild->SendStartPluginIME(aKeyboardEvent, aPanelX,
                                      aPanelY, &aCommitted)) {
     return NS_ERROR_FAILURE;

--- a/widget/PuppetWidget.h
+++ b/widget/PuppetWidget.h
@@ -216,19 +216,20 @@ public:
   // Get the size of the chrome of the window that this tab belongs to.
   nsIntPoint GetChromeDimensions();
 
   // Get the screen position of the application window.
   nsIntPoint GetWindowPosition();
 
   virtual LayoutDeviceIntRect GetScreenBounds() override;
 
-  NS_IMETHOD StartPluginIME(const mozilla::WidgetKeyboardEvent& aKeyboardEvent,
-                            int32_t aPanelX, int32_t aPanelY,
-                            nsString& aCommitted) override;
+  virtual MOZ_MUST_USE nsresult
+  StartPluginIME(const mozilla::WidgetKeyboardEvent& aKeyboardEvent,
+                 int32_t aPanelX, int32_t aPanelY,
+                 nsString& aCommitted) override;
 
   virtual void SetPluginFocused(bool& aFocused) override;
   virtual void DefaultProcOfPluginEvent(
                  const mozilla::WidgetPluginEvent& aEvent) override;
 
   virtual nsresult SynthesizeNativeKeyEvent(int32_t aNativeKeyboardLayout,
                                             int32_t aNativeKeyCode,
                                             uint32_t aModifierFlags,

--- a/widget/cocoa/nsChildView.h
+++ b/widget/cocoa/nsChildView.h
@@ -503,19 +503,20 @@ public:
     StartRemoteDrawingInRegion(LayoutDeviceIntRegion& aInvalidRegion,
                                mozilla::layers::BufferMode* aBufferMode) override;
   void EndRemoteDrawing() override;
   void CleanupRemoteDrawing() override;
   bool InitCompositor(mozilla::layers::Compositor* aCompositor) override;
 
   IAPZCTreeManager* APZCTM() { return mAPZC ; }
 
-  NS_IMETHOD StartPluginIME(const mozilla::WidgetKeyboardEvent& aKeyboardEvent,
-                            int32_t aPanelX, int32_t aPanelY,
-                            nsString& aCommitted) override;
+  virtual MOZ_MUST_USE nsresult
+  StartPluginIME(const mozilla::WidgetKeyboardEvent& aKeyboardEvent,
+                 int32_t aPanelX, int32_t aPanelY,
+                 nsString& aCommitted) override;
 
   virtual void SetPluginFocused(bool& aFocused) override;
 
   bool IsPluginFocused() { return mPluginFocused; }
 
   virtual LayoutDeviceIntPoint GetClientOffset() override;
 
   void DispatchAPZWheelInputEvent(mozilla::InputData& aEvent, bool aCanTriggerSwipe);

--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -1674,17 +1674,17 @@ void nsChildView::UpdateCurrentInputEven
 
 bool nsChildView::HasPendingInputEvent()
 {
   return DoHasPendingInputEvent();
 }
 
 #pragma mark -
 
-NS_IMETHODIMP
+nsresult
 nsChildView::StartPluginIME(const mozilla::WidgetKeyboardEvent& aKeyboardEvent,
                             int32_t aPanelX, int32_t aPanelY,
                             nsString& aCommitted)
 {
   NS_ENSURE_TRUE(mView, NS_ERROR_NOT_AVAILABLE);
 
   ComplexTextInputPanel* ctiPanel =
     ComplexTextInputPanel::GetSharedComplexTextInputPanel();

--- a/widget/nsBaseWidget.h
+++ b/widget/nsBaseWidget.h
@@ -259,17 +259,18 @@ public:
                                           int32_t aVertical) override
                           { return NS_ERROR_NOT_IMPLEMENTED; }
   virtual MOZ_MUST_USE nsresult
                           BeginMoveDrag(mozilla::WidgetMouseEvent* aEvent) override
                           { return NS_ERROR_NOT_IMPLEMENTED; }
   virtual nsresult        ActivateNativeMenuItemAt(const nsAString& indexString) override { return NS_ERROR_NOT_IMPLEMENTED; }
   virtual nsresult        ForceUpdateNativeMenuAt(const nsAString& indexString) override { return NS_ERROR_NOT_IMPLEMENTED; }
   NS_IMETHOD              NotifyIME(const IMENotification& aIMENotification) override final;
-  NS_IMETHOD              StartPluginIME(const mozilla::WidgetKeyboardEvent& aKeyboardEvent,
+  virtual MOZ_MUST_USE nsresult
+                          StartPluginIME(const mozilla::WidgetKeyboardEvent& aKeyboardEvent,
                                          int32_t aPanelX, int32_t aPanelY,
                                          nsString& aCommitted) override
                           { return NS_ERROR_NOT_IMPLEMENTED; }
   virtual void            SetPluginFocused(bool& aFocused) override {}
   virtual void            SetCandidateWindowForPlugin(
                             const mozilla::widget::CandidateWindowPosition&
                               aPosition) override
                           { }

--- a/widget/nsIWidget.h
+++ b/widget/nsIWidget.h
@@ -1749,19 +1749,20 @@ public:
      * Start plugin IME.  If this results in a string getting committed, the
      * result is in aCommitted (otherwise aCommitted is empty).
      *
      * aKeyboardEvent     The event with which plugin IME is to be started
      * panelX and panelY  Location in screen coordinates of the IME input panel
      *                    (should be just under the plugin)
      * aCommitted         The string committed during IME -- otherwise empty
      */
-    NS_IMETHOD StartPluginIME(const mozilla::WidgetKeyboardEvent& aKeyboardEvent,
-                              int32_t aPanelX, int32_t aPanelY,
-                              nsString& aCommitted) = 0;
+    virtual MOZ_MUST_USE nsresult
+    StartPluginIME(const mozilla::WidgetKeyboardEvent& aKeyboardEvent,
+                   int32_t aPanelX, int32_t aPanelY,
+                   nsString& aCommitted) = 0;
 
     /**
      * Tells the widget whether or not a plugin (inside the widget) has the
      * keyboard focus.  Should be sent when the keyboard focus changes too or
      * from a plugin.
      *
      * aFocused  Whether or not a plugin is focused
      */