diff --git a/widget/cocoa/nsCocoaWindow.h b/widget/cocoa/nsCocoaWindow.h
index cc7660fc9240..ce895c60a6cd 100644
--- a/widget/cocoa/nsCocoaWindow.h
+++ b/widget/cocoa/nsCocoaWindow.h
@@ -285,7 +285,13 @@ public:
                                         nsIWidget *aWidget, bool aActivate) override;
     NS_IMETHOD              SetSizeMode(int32_t aMode) override;
     NS_IMETHOD              HideWindowChrome(bool aShouldHide) override;
+
     void EnteredFullScreen(bool aFullScreen, bool aNativeMode = true);
+    virtual bool PrepareForFullscreenTransition(nsISupports** aData) override;
+    virtual void PerformFullscreenTransition(FullscreenTransitionStage aStage,
+                                             uint16_t aDuration,
+                                             nsISupports* aData,
+                                             nsIRunnable* aCallback) override;
     NS_IMETHOD MakeFullScreen(
       bool aFullScreen, nsIScreen* aTargetScreen = nullptr) override final;
     NS_IMETHOD MakeFullScreenWithNativeTransition(
diff --git a/widget/cocoa/nsCocoaWindow.mm b/widget/cocoa/nsCocoaWindow.mm
index 0a570f174a86..ca22ad837064 100644
--- a/widget/cocoa/nsCocoaWindow.mm
+++ b/widget/cocoa/nsCocoaWindow.mm
@@ -1,4 +1,5 @@
 /* -*- Mode: Objective-C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
@@ -33,6 +34,7 @@
 #include "nsIScreenManager.h"
 #include "nsIWidgetListener.h"
 #include "nsIPresShell.h"
+#include "nsScreenCocoa.h"
 
 #include "gfxPlatform.h"
 #include "qcms.h"
@@ -1265,6 +1267,90 @@ NS_IMETHODIMP nsCocoaWindow::HideWindowChrome(bool aShouldHide)
   NS_OBJC_END_TRY_ABORT_BLOCK_NSRESULT;
 }
 
+class FullscreenTransitionData : public nsISupports
+{
+public:
+  NS_DECL_ISUPPORTS
+
+  explicit FullscreenTransitionData(NSWindow* aWindow)
+    : mTransitionWindow(aWindow) { }
+
+  NSWindow* mTransitionWindow;
+
+private:
+  virtual ~FullscreenTransitionData()
+  {
+    [mTransitionWindow close];
+  }
+};
+
+NS_IMPL_ISUPPORTS0(FullscreenTransitionData)
+
+@interface FullscreenTransitionDelegate : NSObject <NSAnimationDelegate>
+{
+@public
+  nsIRunnable* mCallback;
+}
+@end
+
+@implementation FullscreenTransitionDelegate
+- (void)animationDidEnd:(NSAnimation *)animation
+{
+  [animation setDelegate:nil];
+  [self autorelease];
+  // The caller should have added ref for us.
+  NS_DispatchToMainThread(already_AddRefed<nsIRunnable>(mCallback));
+}
+@end
+
+/* virtual */ bool
+nsCocoaWindow::PrepareForFullscreenTransition(nsISupports** aData)
+{
+  nsCOMPtr<nsIScreen> widgetScreen = GetWidgetScreen();
+  nsScreenCocoa* screen = static_cast<nsScreenCocoa*>(widgetScreen.get());
+  NSScreen* cocoaScreen = screen->CocoaScreen();
+
+  NSWindow* win =
+    [[NSWindow alloc] initWithContentRect:[cocoaScreen frame]
+                                styleMask:NSBorderlessWindowMask
+                                  backing:NSBackingStoreBuffered
+                                    defer:YES];
+  [win setBackgroundColor:[NSColor blackColor]];
+  [win setAlphaValue:0];
+  [win setIgnoresMouseEvents:YES];
+  [win setLevel:NSScreenSaverWindowLevel];
+  [win makeKeyAndOrderFront:nil];
+
+  auto data = new FullscreenTransitionData(win);
+  *aData = data;
+  NS_ADDREF(data);
+  return true;
+}
+
+/* virtual */ void
+nsCocoaWindow::PerformFullscreenTransition(FullscreenTransitionStage aStage,
+                                           uint16_t aDuration,
+                                           nsISupports* aData,
+                                           nsIRunnable* aCallback)
+{
+  auto data = static_cast<FullscreenTransitionData*>(aData);
+  FullscreenTransitionDelegate* delegate =
+    [[FullscreenTransitionDelegate alloc] init];
+  // Storing already_AddRefed directly could cause static checking fail.
+  delegate->mCallback = nsCOMPtr<nsIRunnable>(aCallback).forget().take();
+
+  NSDictionary* dict = @{
+    NSViewAnimationTargetKey: data->mTransitionWindow,
+    NSViewAnimationEffectKey: aStage == eBeforeFullscreenToggle ?
+      NSViewAnimationFadeInEffect : NSViewAnimationFadeOutEffect
+  };
+  NSAnimation* animation =
+    [[[NSViewAnimation alloc] initWithViewAnimations:@[dict]] autorelease];
+  [animation setDelegate:delegate];
+  [animation setDuration:aDuration / 1000.0];
+  [animation startAnimation];
+}
+
 void nsCocoaWindow::EnteredFullScreen(bool aFullScreen, bool aNativeMode)
 {
   mInFullScreenTransition = false;