--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -11553,16 +11553,58 @@ nsresult nsDocument::RemoteFrameFullscre
 static void
 ReleaseVRDeviceProxyRef(void *, nsIAtom*, void *aPropertyValue, void *)
 {
   if (aPropertyValue) {
     static_cast<gfx::VRDeviceProxy*>(aPropertyValue)->Release();
   }
 }
 
+static bool
+HasFullScreenSubDocument(nsIDocument* aDoc)
+{
+  uint32_t count = CountFullscreenSubDocuments(aDoc);
+  NS_ASSERTION(count <= 1, "Fullscreen docs should have at most 1 fullscreen child!");
+  return count >= 1;
+}
+
+// Returns nullptr if a request for Fullscreen API is currently enabled
+// in the given document. Returns a static string indicates the reason
+// why it is not enabled otherwise.
+static const char*
+GetFullscreenError(nsIDocument* aDoc, bool aCallerIsChrome)
+{
+  if (nsContentUtils::IsFullScreenApiEnabled() && aCallerIsChrome) {
+    // Chrome code can always use the full-screen API, provided it's not
+    // explicitly disabled. Note IsCallerChrome() returns true when running
+    // in an nsRunnable, so don't use GetMozFullScreenEnabled() from an
+    // nsRunnable!
+    return nullptr;
+  }
+
+  if (!nsContentUtils::IsFullScreenApiEnabled()) {
+    return "FullScreenDeniedDisabled";
+  }
+  if (!aDoc->IsVisible()) {
+    return "FullScreenDeniedHidden";
+  }
+  if (HasFullScreenSubDocument(aDoc)) {
+    return "FullScreenDeniedSubDocFullScreen";
+  }
+
+  // Ensure that all containing elements are <iframe> and have
+  // allowfullscreen attribute set.
+  nsCOMPtr<nsIDocShell> docShell(aDoc->GetDocShell());
+  if (!docShell || !docShell->GetFullscreenAllowed()) {
+    return "FullScreenDeniedContainerNotAllowed";
+  }
+
+  return nullptr;
+}
+
 bool
 nsDocument::FullscreenElementReadyCheck(Element* aElement,
                                         bool aWasCallerChrome)
 {
   NS_ASSERTION(aElement,
     "Must pass non-null element to nsDocument::RequestFullScreen");
   if (!aElement || aElement == GetFullscreenElement()) {
     return false;
@@ -11574,18 +11616,18 @@ nsDocument::FullscreenElementReadyCheck(
   if (aElement->OwnerDoc() != this) {
     LogFullScreenDenied(true, "FullScreenDeniedMovedDocument", this);
     return false;
   }
   if (!GetWindow()) {
     LogFullScreenDenied(true, "FullScreenDeniedLostWindow", this);
     return false;
   }
-  if (!IsFullScreenEnabled(aWasCallerChrome, true)) {
-    // IsFullScreenEnabled calls LogFullScreenDenied, no need to log.
+  if (const char* msg = GetFullscreenError(this, aWasCallerChrome)) {
+    LogFullScreenDenied(true, msg, this);
     return false;
   }
   if (GetFullscreenElement() &&
       !nsContentUtils::ContentIsDescendantOf(aElement, GetFullscreenElement())) {
     // If this document is full-screen, only grant full-screen requests from
     // a descendant of the current full-screen element.
     LogFullScreenDenied(true, "FullScreenDeniedNotDescendant", this);
     return false;
@@ -11984,60 +12026,17 @@ nsDocument::GetMozFullScreenEnabled(bool
   NS_ENSURE_ARG_POINTER(aFullScreen);
   *aFullScreen = FullscreenEnabled();
   return NS_OK;
 }
 
 bool
 nsDocument::FullscreenEnabled()
 {
-  return IsFullScreenEnabled(nsContentUtils::IsCallerChrome(), false);
-}
-
-static bool
-HasFullScreenSubDocument(nsIDocument* aDoc)
-{
-  uint32_t count = CountFullscreenSubDocuments(aDoc);
-  NS_ASSERTION(count <= 1, "Fullscreen docs should have at most 1 fullscreen child!");
-  return count >= 1;
-}
-
-bool
-nsDocument::IsFullScreenEnabled(bool aCallerIsChrome, bool aLogFailure)
-{
-  if (nsContentUtils::IsFullScreenApiEnabled() && aCallerIsChrome) {
-    // Chrome code can always use the full-screen API, provided it's not
-    // explicitly disabled. Note IsCallerChrome() returns true when running
-    // in an nsRunnable, so don't use GetMozFullScreenEnabled() from an
-    // nsRunnable!
-    return true;
-  }
-
-  if (!nsContentUtils::IsFullScreenApiEnabled()) {
-    LogFullScreenDenied(aLogFailure, "FullScreenDeniedDisabled", this);
-    return false;
-  }
-  if (!IsVisible()) {
-    LogFullScreenDenied(aLogFailure, "FullScreenDeniedHidden", this);
-    return false;
-  }
-  if (HasFullScreenSubDocument(this)) {
-    LogFullScreenDenied(aLogFailure, "FullScreenDeniedSubDocFullScreen", this);
-    return false;
-  }
-
-  // Ensure that all containing elements are <iframe> and have
-  // allowfullscreen attribute set.
-  nsCOMPtr<nsIDocShell> docShell(mDocumentContainer);
-  if (!docShell || !docShell->GetFullscreenAllowed()) {
-    LogFullScreenDenied(aLogFailure, "FullScreenDeniedContainerNotAllowed", this);
-    return false;
-  }
-
-  return true;
+  return !GetFullscreenError(this, nsContentUtils::IsCallerChrome());
 }
 
 uint16_t
 nsDocument::CurrentOrientationAngle() const
 {
   return mCurrentOrientationAngle;
 }
 

--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -1413,25 +1413,16 @@ protected:
   void DecreaseStackRefCnt()
   {
     if (--mStackRefCnt == 0 && mNeedsReleaseAfterStackRefCntRelease) {
       mNeedsReleaseAfterStackRefCntRelease = false;
       NS_RELEASE_THIS();
     }
   }
 
-  // Returns true if a request for DOM full-screen is currently enabled in
-  // this document. This returns true if there are no windowed plugins in this
-  // doc tree, and if the document is visible, and if the api is not
-  // disabled by pref. aIsCallerChrome must contain the return value of
-  // nsContentUtils::IsCallerChrome() from the context we're checking.
-  // If aLogFailure is true, an appropriate warning message is logged to the
-  // console, and a "mozfullscreenerror" event is dispatched to this document.
-  bool IsFullScreenEnabled(bool aIsCallerChrome, bool aLogFailure);
-
   /**
    * Check that aId is not empty and log a message to the console
    * service if it is.
    * @returns true if aId looks correct, false otherwise.
    */
   inline bool CheckGetElementByIdArg(const nsAString& aId)
   {
     if (aId.IsEmpty()) {