--- a/content/base/src/nsGkAtomList.h
+++ b/content/base/src/nsGkAtomList.h
@@ -58,16 +58,17 @@
 #endif 
 
 //---------------------------------------------------------------------------
 // Generic atoms
 //---------------------------------------------------------------------------
 
 GK_ATOM(_empty, "")
 GK_ATOM(moz, "_moz")
+GK_ATOM(mozallowfullscreen, "mozallowfullscreen")
 GK_ATOM(moztype, "_moz-type")
 GK_ATOM(mozdirty, "_moz_dirty")
 GK_ATOM(mozdonotsend, "moz-do-not-send")
 GK_ATOM(mozeditorbogusnode, "_moz_editor_bogus_node")
 GK_ATOM(mozgeneratedcontentbefore, "_moz_generated_content_before")
 GK_ATOM(mozgeneratedcontentafter, "_moz_generated_content_after")
 GK_ATOM(mozgeneratedcontentimage, "_moz_generated_content_image")
 GK_ATOM(_moz_original_size, "_moz_original_size")

--- a/content/html/content/src/nsGenericHTMLElement.cpp
+++ b/content/html/content/src/nsGenericHTMLElement.cpp
@@ -3380,16 +3380,31 @@ nsGenericHTMLElement::Focus()
 }
 
 nsresult nsGenericHTMLElement::MozRequestFullScreen()
 {
   if (!nsContentUtils::IsFullScreenApiEnabled()) {
     return NS_OK;
   }
 
+  // Ensure that all ancestor <iframe> elements have the mozallowfullscreen
+  // boolean attribute set.
+  nsINode* node = static_cast<nsINode*>(this);
+  do {
+    nsIContent* content = static_cast<nsIContent*>(node);
+    if (content->IsHTML(nsGkAtoms::iframe) &&
+        !content->HasAttr(kNameSpaceID_None, nsGkAtoms::mozallowfullscreen)) {
+      // The node requesting fullscreen, or one of its crossdoc ancestors,
+      // is an iframe which doesn't have the "mozalllowfullscreen" attribute.
+      // This request is not authorized by the parent document.
+      return NS_OK;
+    }
+    node = nsContentUtils::GetCrossDocParentNode(node);
+  } while (node);
+
   nsIDocument* doc = GetOwnerDoc();
   NS_ENSURE_STATE(doc);
   doc->RequestFullScreen(this);
 #ifdef DEBUG
   nsCOMPtr<nsPIDOMWindow> window = do_QueryInterface(doc->GetWindow());
   NS_ENSURE_STATE(window);
   PRBool fullscreen;
   window->GetFullScreen(&fullscreen);

--- a/content/html/content/src/nsHTMLIFrameElement.cpp
+++ b/content/html/content/src/nsHTMLIFrameElement.cpp
@@ -126,16 +126,17 @@ NS_IMPL_STRING_ATTR(nsHTMLIFrameElement,
 NS_IMPL_STRING_ATTR(nsHTMLIFrameElement, Height, height)
 NS_IMPL_URI_ATTR(nsHTMLIFrameElement, LongDesc, longdesc)
 NS_IMPL_STRING_ATTR(nsHTMLIFrameElement, MarginHeight, marginheight)
 NS_IMPL_STRING_ATTR(nsHTMLIFrameElement, MarginWidth, marginwidth)
 NS_IMPL_STRING_ATTR(nsHTMLIFrameElement, Name, name)
 NS_IMPL_STRING_ATTR(nsHTMLIFrameElement, Scrolling, scrolling)
 NS_IMPL_URI_ATTR(nsHTMLIFrameElement, Src, src)
 NS_IMPL_STRING_ATTR(nsHTMLIFrameElement, Width, width)
+NS_IMPL_BOOL_ATTR(nsHTMLIFrameElement, MozAllowFullScreen, mozallowfullscreen)
 
 NS_IMETHODIMP
 nsHTMLIFrameElement::GetContentDocument(nsIDOMDocument** aContentDocument)
 {
   return nsGenericHTMLFrameElement::GetContentDocument(aContentDocument);
 }
 
 NS_IMETHODIMP

--- a/dom/interfaces/html/nsIDOMHTMLIFrameElement.idl
+++ b/dom/interfaces/html/nsIDOMHTMLIFrameElement.idl
@@ -45,24 +45,29 @@
  *
  * This interface is trying to follow the DOM Level 2 HTML specification:
  * http://www.w3.org/TR/DOM-Level-2-HTML/
  *
  * with changes from the work-in-progress WHATWG HTML specification:
  * http://www.whatwg.org/specs/web-apps/current-work/
  */
 
-[scriptable, uuid(33dfbcdc-4edf-4e6a-acf4-c6b5bbb61caf)]
+[scriptable, uuid(31969911-36fb-42ee-9a00-018c3ff78cfb)]
 interface nsIDOMHTMLIFrameElement : nsIDOMHTMLElement
 {
            attribute DOMString        align;
            attribute DOMString        frameBorder;
            attribute DOMString        height;
            attribute DOMString        longDesc;
            attribute DOMString        marginHeight;
            attribute DOMString        marginWidth;
            attribute DOMString        name;
            attribute DOMString        scrolling;
            attribute DOMString        src;
            attribute DOMString        width;
   // Introduced in DOM Level 2:
   readonly attribute nsIDOMDocument   contentDocument;
+
+  // iframe elements require the mozAllowFullScreen attribute to be present
+  // if they're to allow content in the sub document to go into DOM full-screen
+  // mode. See https://wiki.mozilla.org/index.php?title=Gecko:FullScreenAPI
+           attribute boolean          mozAllowFullScreen;
 };