diff --git a/widget/cocoa/nsCocoaWindow.h b/widget/cocoa/nsCocoaWindow.h
index f01aebe26973..c9450d9d652d 100644
--- a/widget/cocoa/nsCocoaWindow.h
+++ b/widget/cocoa/nsCocoaWindow.h
@@ -53,6 +53,21 @@ class nsCocoaWindow;
 class nsChildView;
 class nsMenuBarX;
 
+// If we are using an SDK older than 10.7, define bits we need that are missing
+// from it.
+#if !defined(MAC_OS_X_VERSION_10_7) || \
+    MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_7
+
+enum {
+    NSWindowCollectionBehaviorFullScreenPrimary = 1 << 7,
+};
+
+@interface NSWindow (NewInMacOS107)
+- (void)toggleFullScreen:(id)sender;
+@end
+
+#endif
+
 typedef struct _nsCocoaWindowList {
   _nsCocoaWindowList() : prev(NULL), window(NULL) {}
   struct _nsCocoaWindowList *prev;
@@ -255,6 +270,7 @@ public:
                                         nsIWidget *aWidget, bool aActivate);
     NS_IMETHOD              SetSizeMode(PRInt32 aMode);
     NS_IMETHOD              HideWindowChrome(bool aShouldHide);
+    void                    EnteredFullScreen(bool aFullScreen);
     NS_IMETHOD              MakeFullScreen(bool aFullScreen);
     NS_IMETHOD              Resize(PRInt32 aWidth,PRInt32 aHeight, bool aRepaint);
     NS_IMETHOD              Resize(PRInt32 aX, PRInt32 aY, PRInt32 aWidth, PRInt32 aHeight, bool aRepaint);
@@ -281,6 +297,7 @@ public:
     virtual void SetTransparencyMode(nsTransparencyMode aMode);
     NS_IMETHOD SetWindowShadowStyle(PRInt32 aStyle);
     virtual void SetShowsToolbarButton(bool aShow);
+    virtual void SetShowsFullScreenButton(bool aShow);
     virtual void SetWindowAnimationType(WindowAnimationType aType);
     NS_IMETHOD SetWindowTitlebarColor(nscolor aColor, bool aActive);
     virtual void SetDrawsInTitlebar(bool aState);
@@ -359,6 +376,8 @@ protected:
   bool                 mFullScreen;
   bool                 mModal;
 
+  bool                 mUsesNativeFullScreen; // only true on Lion if SetShowsFullScreenButton(true);
+
   bool                 mIsAnimationSuppressed;
 
   bool                 mInReportMoveEvent; // true if in a call to ReportMoveEvent().
diff --git a/widget/cocoa/nsCocoaWindow.mm b/widget/cocoa/nsCocoaWindow.mm
index 7175679bc08a..80fc621d7f66 100644
--- a/widget/cocoa/nsCocoaWindow.mm
+++ b/widget/cocoa/nsCocoaWindow.mm
@@ -140,6 +140,7 @@ nsCocoaWindow::nsCocoaWindow()
 , mSheetNeedsShow(false)
 , mFullScreen(false)
 , mModal(false)
+, mUsesNativeFullScreen(false)
 , mIsAnimationSuppressed(false)
 , mInReportMoveEvent(false)
 , mNumModalDescendents(0)
@@ -513,7 +514,10 @@ NS_IMETHODIMP nsCocoaWindow::Destroy()
   nsBaseWidget::Destroy();
   nsBaseWidget::OnDestroy();
 
-  if (mFullScreen) {
+  // On Lion we do not have to mess with the OS chrome when in Full Screen mode. So we
+  // can simply skip that. When the Window is destroyed, the OS will take care of removing
+  // the full screen 'Space' that was setup for us.
+  if (!mUsesNativeFullScreen && mFullScreen) {
     nsCocoaUtils::HideOSChromeOnScreen(false, [mWindow screen]);
   }
 
@@ -1180,24 +1184,40 @@ NS_IMETHODIMP nsCocoaWindow::HideWindowChrome(bool aShouldHide)
   NS_OBJC_END_TRY_ABORT_BLOCK_NSRESULT;
 }
 
+void nsCocoaWindow::EnteredFullScreen(bool aFullScreen)
+{
+  mFullScreen = aFullScreen;
+  DispatchSizeModeEvent();
+}
 
 NS_METHOD nsCocoaWindow::MakeFullScreen(bool aFullScreen)
 {
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK_NSRESULT;
 
-  NS_ASSERTION(mFullScreen != aFullScreen, "Unnecessary MakeFullScreen call");
+  // We will call into MakeFullScreen redundantly when entering/exiting
+  // fullscreen mode via OS X controls. When that happens we should just handle
+  // it gracefully - no need to ASSERT.
+  if (mFullScreen == aFullScreen) {
+    return NS_OK;
+  }
 
-  NSDisableScreenUpdates();
-  // The order here matters. When we exit full screen mode, we need to show the
-  // Dock first, otherwise the newly-created window won't have its minimize
-  // button enabled. See bug 526282.
-  nsCocoaUtils::HideOSChromeOnScreen(aFullScreen, [mWindow screen]);
-  nsresult rv = nsBaseWidget::MakeFullScreen(aFullScreen);
-  NSEnableScreenUpdates();
-  NS_ENSURE_SUCCESS(rv, rv);
+  if (mUsesNativeFullScreen) {
+    // Calling toggleFullScreen will result in windowDid(Enter|Exit)FullScreen
+    // to be called from the OS. We will call EnteredFullScreen from those methods,
+    // where mFullScreen will be set and a sizemode event will be dispatched.
+    [mWindow toggleFullScreen:nil];
+  } else {
+    NSDisableScreenUpdates();
+    // The order here matters. When we exit full screen mode, we need to show the
+    // Dock first, otherwise the newly-created window won't have its minimize
+    // button enabled. See bug 526282.
+    nsCocoaUtils::HideOSChromeOnScreen(aFullScreen, [mWindow screen]);
+    nsresult rv = nsBaseWidget::MakeFullScreen(aFullScreen);
+    NSEnableScreenUpdates();
+    NS_ENSURE_SUCCESS(rv, rv);
 
-  mFullScreen = aFullScreen;
-  DispatchSizeModeEvent();
+    EnteredFullScreen(aFullScreen);
+  }
 
   return NS_OK;
 
@@ -1642,6 +1662,42 @@ void nsCocoaWindow::SetShowsToolbarButton(bool aShow)
   NS_OBJC_END_TRY_ABORT_BLOCK;
 }
 
+void nsCocoaWindow::SetShowsFullScreenButton(bool aShow)
+{
+  NS_OBJC_BEGIN_TRY_ABORT_BLOCK;
+
+  if (![mWindow respondsToSelector:@selector(toggleFullScreen:)] ||
+      mUsesNativeFullScreen == aShow) {
+    return;
+  }
+
+  // If the window is currently in fullscreen mode, then we're going to
+  // transition out first, then set the collection behavior & toggle
+  // mUsesNativeFullScreen, then transtion back into fullscreen mode. This
+  // prevents us from getting into a conflicting state with MakeFullScreen
+  // where mUsesNativeFullScreen would lead us down the wrong path.
+  bool wasFullScreen = mFullScreen;
+
+  if (wasFullScreen) {
+    MakeFullScreen(false);
+  }
+
+  NSWindowCollectionBehavior newBehavior = [mWindow collectionBehavior];
+  if (aShow) {
+    newBehavior |= NSWindowCollectionBehaviorFullScreenPrimary;
+  } else {
+    newBehavior &= ~NSWindowCollectionBehaviorFullScreenPrimary;
+  }
+  [mWindow setCollectionBehavior:newBehavior];
+  mUsesNativeFullScreen = aShow;
+
+  if (wasFullScreen) {
+    MakeFullScreen(true);
+  }
+
+  NS_OBJC_END_TRY_ABORT_BLOCK;
+}
+
 void nsCocoaWindow::SetWindowAnimationType(nsIWidget::WindowAnimationType aType)
 {
   mAnimationType = aType;
@@ -1875,6 +1931,27 @@ bool nsCocoaWindow::ShouldFocusPlugin()
   mGeckoWindow->ReportMoveEvent();
 }
 
+// Lion's full screen mode will bypass our internal fullscreen tracking, so
+// we need to catch it when we transition and call our own methods, which in
+// turn will fire "fullscreen" events.
+- (void)windowDidEnterFullScreen:(NSNotification *)notification
+{
+  if (!mGeckoWindow) {
+    return;
+  }
+
+  mGeckoWindow->EnteredFullScreen(true);
+}
+
+- (void)windowDidExitFullScreen:(NSNotification *)notification
+{
+  if (!mGeckoWindow) {
+    return;
+  }
+
+  mGeckoWindow->EnteredFullScreen(false);
+}
+
 - (void)windowDidBecomeMain:(NSNotification *)aNotification
 {
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK;