diff --git a/content/base/src/nsDocument.cpp b/content/base/src/nsDocument.cpp
index e908dc4476f2..d8ca57634538 100644
--- a/content/base/src/nsDocument.cpp
+++ b/content/base/src/nsDocument.cpp
@@ -8532,7 +8532,7 @@ public:
   NS_IMETHOD Run()
   {
     if (mDoc->GetWindow()) {
-      mDoc->GetWindow()->SetFullScreen(mValue);
+      mDoc->GetWindow()->SetFullScreenInternal(mValue, false);
     }
     return NS_OK;
   }
diff --git a/content/html/content/test/test_fullscreen-api.html b/content/html/content/test/test_fullscreen-api.html
index e65da921d1de..fe84d0d4ca8e 100644
--- a/content/html/content/test/test_fullscreen-api.html
+++ b/content/html/content/test/test_fullscreen-api.html
@@ -70,6 +70,12 @@ function nextTest() {
   }
 }
 
+try {
+  window.fullScreen = true;
+} catch (e) {
+}
+is(window.fullScreen, false, "Shouldn't be able to set window fullscreen from content");
+
 addLoadEvent(nextTest);
 SimpleTest.waitForExplicitFinish();
 
diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index 589f9a0c893e..02592097aaf1 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -4457,6 +4457,12 @@ nsGlobalWindow::GetNearestWidget()
 
 NS_IMETHODIMP
 nsGlobalWindow::SetFullScreen(bool aFullScreen)
+{
+  return SetFullScreenInternal(aFullScreen, true);
+}
+
+nsresult
+nsGlobalWindow::SetFullScreenInternal(bool aFullScreen, bool aRequireTrust)
 {
   FORWARD_TO_OUTER(SetFullScreen, (aFullScreen), NS_ERROR_NOT_INITIALIZED);
 
@@ -4464,11 +4470,10 @@ nsGlobalWindow::SetFullScreen(bool aFullScreen)
 
   bool rootWinFullScreen;
   GetFullScreen(&rootWinFullScreen);
-  // Only chrome can change our fullScreen mode, unless the DOM full-screen
-  // API is enabled.
-  if ((aFullScreen == rootWinFullScreen || 
-      !nsContentUtils::IsCallerTrustedForWrite()) &&
-      !nsContentUtils::IsFullScreenApiEnabled()) {
+  // Only chrome can change our fullScreen mode, unless we're running in
+  // untrusted mode.
+  if (aFullScreen == rootWinFullScreen || 
+      (aRequireTrust && !nsContentUtils::IsCallerTrustedForWrite())) {
     return NS_OK;
   }
 
@@ -4478,11 +4483,11 @@ nsGlobalWindow::SetFullScreen(bool aFullScreen)
   nsCOMPtr<nsIDocShellTreeItem> treeItem = do_QueryInterface(mDocShell);
   nsCOMPtr<nsIDocShellTreeItem> rootItem;
   treeItem->GetRootTreeItem(getter_AddRefs(rootItem));
-  nsCOMPtr<nsIDOMWindow> window = do_GetInterface(rootItem);
+  nsCOMPtr<nsPIDOMWindow> window = do_GetInterface(rootItem);
   if (!window)
     return NS_ERROR_FAILURE;
   if (rootItem != treeItem)
-    return window->SetFullScreen(aFullScreen);
+    return window->SetFullScreenInternal(aFullScreen, aRequireTrust);
 
   // make sure we don't try to set full screen on a non-chrome window,
   // which might happen in embedding world
diff --git a/dom/base/nsGlobalWindow.h b/dom/base/nsGlobalWindow.h
index a145c86e87c7..6b254237400d 100644
--- a/dom/base/nsGlobalWindow.h
+++ b/dom/base/nsGlobalWindow.h
@@ -398,6 +398,7 @@ public:
   virtual NS_HIDDEN_(void) MaybeUpdateTouchState();
   virtual NS_HIDDEN_(void) UpdateTouchState();
   virtual NS_HIDDEN_(bool) DispatchCustomEvent(const char *aEventName);
+  virtual NS_HIDDEN_(nsresult) SetFullScreenInternal(bool aIsFullScreen, bool aRequireTrust);
 
   // nsIDOMStorageIndexedDB
   NS_DECL_NSIDOMSTORAGEINDEXEDDB
diff --git a/dom/base/nsPIDOMWindow.h b/dom/base/nsPIDOMWindow.h
index 18d11380719f..fdfceabc72d2 100644
--- a/dom/base/nsPIDOMWindow.h
+++ b/dom/base/nsPIDOMWindow.h
@@ -80,8 +80,8 @@ class nsIArray;
 class nsPIWindowRoot;
 
 #define NS_PIDOMWINDOW_IID \
-{ 0x1352de12, 0x7a07, 0x4610, \
-  { 0x93, 0xd5, 0xb8, 0x76, 0xfe, 0x93, 0x09, 0x50 } }
+{ 0x9aef58e9, 0x5225, 0x4e58, \
+  { 0x9a, 0xfb, 0xe6, 0x63, 0x97, 0x1d, 0x86, 0x88 } }
 
 class nsPIDOMWindow : public nsIDOMWindowInternal
 {
@@ -457,6 +457,13 @@ public:
     return mMayHaveTouchEventListener;
   }
 
+  /**
+   * Moves the top-level window into fullscreen mode if aIsFullScreen is true,
+   * otherwise exits fullscreen. If aRequireTrust is true, this method only
+   * changes window state in a context trusted for write.
+   */
+  virtual nsresult SetFullScreenInternal(bool aIsFullScreen, bool aRequireTrust) = 0;
+
   /**
    * Call this to indicate that some node (this window, its document,
    * or content in that document) has a "MozAudioAvailable" event listener.