diff --git a/widget/cocoa/nsCocoaWindow.mm b/widget/cocoa/nsCocoaWindow.mm
index 0b84bf06f7b..cd1f68e6c61 100644
--- a/widget/cocoa/nsCocoaWindow.mm
+++ b/widget/cocoa/nsCocoaWindow.mm
@@ -2834,6 +2834,37 @@ void nsCocoaWindow::CocoaWindowDidResize() {
   mGeckoWindow->CocoaWindowDidEnterFullscreen(false);
 }
 
+- (void)windowDidFailToEnterFullScreen:(NSNotification*)notification {
+  if (!mGeckoWindow) {
+    return;
+  }
+
+  MOZ_ASSERT((mGeckoWindow->GetCocoaWindow().styleMask &
+              NSWindowStyleMaskFullScreen) == 0);
+  MOZ_ASSERT(mGeckoWindow->SizeMode() == nsSizeMode_Fullscreen);
+
+  // We're in a strange situation. We've told DOM that we are going to
+  // fullscreen by changing our size mode, and therefore the window
+  // content is what we would show if we were properly in fullscreen.
+  // But the window is actually in a windowed style. We have to do
+  // several things:
+  // 1) Clear sWindowInNativeTransition and mTransitionCurrent, both set
+  //    when we started the fullscreen transition.
+  // 2) Change our size mode to windowed.
+  // Conveniently, we can do these things by pretending we just arrived
+  // at windowed mode, and all will be sorted out.
+  mGeckoWindow->CocoaWindowDidEnterFullscreen(false);
+}
+
+- (void)windowDidFailToExitFullScreen:(NSNotification*)notification {
+  if (!mGeckoWindow) {
+    return;
+  }
+  // Similarly to windowDidFailToEnterFullScreen, we can get the right
+  // result by pretending we just entered fullscreen.
+  mGeckoWindow->CocoaWindowDidEnterFullscreen(true);
+}
+
 - (void)windowDidBecomeMain:(NSNotification*)aNotification {
   NS_OBJC_BEGIN_TRY_IGNORE_BLOCK;