diff --git a/widget/cocoa/nsChildView.h b/widget/cocoa/nsChildView.h
index b9808c4ed1d..3f4f6ab623a 100644
--- a/widget/cocoa/nsChildView.h
+++ b/widget/cocoa/nsChildView.h
@@ -185,6 +185,10 @@ enum {
 };
 typedef NSUInteger NSEventPhase;
 
+enum {
+   NSFullScreenWindowMask = 1 << 14
+};
+
 @interface NSWindow (LionWindowFeatures)
 - (NSRect)convertRectToScreen:(NSRect)aRect;
 @end
@@ -681,6 +685,7 @@ protected:
   bool mHasRoundedBottomCorners;
   int mDevPixelCornerRadius;
   bool mIsCoveringTitlebar;
+  bool mIsFullscreen;
   nsIntRect mTitlebarRect;
 
   // The area of mTitlebarCGContext that needs to be redrawn during the next
diff --git a/widget/cocoa/nsChildView.mm b/widget/cocoa/nsChildView.mm
index aac79a39ec7..d87a4e133b5 100644
--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -376,6 +376,7 @@ nsChildView::nsChildView() : nsBaseWidget()
 , mShowsResizeIndicator(false)
 , mHasRoundedBottomCorners(false)
 , mIsCoveringTitlebar(false)
+, mIsFullscreen(false)
 , mTitlebarCGContext(nullptr)
 , mBackingScaleFactor(0.0)
 , mVisible(false)
@@ -2071,6 +2072,7 @@ nsChildView::PrepareWindowEffects()
   CGFloat cornerRadius = [(ChildView*)mView cornerRadius];
   mDevPixelCornerRadius = cornerRadius * BackingScaleFactor();
   mIsCoveringTitlebar = [(ChildView*)mView isCoveringTitlebar];
+  mIsFullscreen = ([[mView window] styleMask] & NSFullScreenWindowMask) != 0;
   if (mIsCoveringTitlebar) {
     mTitlebarRect = RectContainingTitlebarControls();
     UpdateTitlebarCGContext();
@@ -2375,7 +2377,7 @@ void
 nsChildView::MaybeDrawTitlebar(GLManager* aManager, const nsIntRect& aRect)
 {
   MutexAutoLock lock(mEffectsLock);
-  if (!mIsCoveringTitlebar) {
+  if (!mIsCoveringTitlebar || mIsFullscreen) {
     return;
   }
 
@@ -2428,13 +2430,13 @@ nsChildView::MaybeDrawRoundedCorners(GLManager* aManager, const nsIntRect& aRect
   gfx3DMatrix flipX = gfx3DMatrix::ScalingMatrix(-1, 1, 1);
   gfx3DMatrix flipY = gfx3DMatrix::ScalingMatrix(1, -1, 1);
 
-  if (mIsCoveringTitlebar) {
+  if (mIsCoveringTitlebar && !mIsFullscreen) {
     // Mask the top corners.
     mCornerMaskImage->Draw(aManager, aRect.TopLeft());
     mCornerMaskImage->Draw(aManager, aRect.TopRight(), flipX);
   }
 
-  if (mHasRoundedBottomCorners) {
+  if (mHasRoundedBottomCorners && !mIsFullscreen) {
     // Mask the bottom corners.
     mCornerMaskImage->Draw(aManager, aRect.BottomLeft(), flipY);
     mCornerMaskImage->Draw(aManager, aRect.BottomRight(), flipY * flipX);