--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -4077,17 +4077,26 @@ nsDocShell::IsPrintingOrPP(bool aDisplay
   }
 
   return mIsPrintingOrPP;
 }
 
 bool
 nsDocShell::IsNavigationAllowed(bool aDisplayPrintErrorDialog)
 {
-    return !IsPrintingOrPP(aDisplayPrintErrorDialog) && !mFiredUnloadEvent;
+  bool isAllowed = !IsPrintingOrPP(aDisplayPrintErrorDialog) && !mFiredUnloadEvent;
+  if (!isAllowed) {
+    return false;
+  }
+  if (!mContentViewer) {
+    return true;
+  }
+  bool firingBeforeUnload;
+  mContentViewer->GetBeforeUnloadFiring(&firingBeforeUnload);
+  return !firingBeforeUnload;
 }
 
 //*****************************************************************************
 // nsDocShell::nsIWebNavigation
 //*****************************************************************************   
 
 NS_IMETHODIMP
 nsDocShell::GetCanGoBack(bool * aCanGoBack)

--- a/docshell/base/nsIContentViewer.idl
+++ b/docshell/base/nsIContentViewer.idl
@@ -23,17 +23,17 @@ class nsDOMNavigationTiming;
 [ptr] native nsIWidgetPtr(nsIWidget);
 [ptr] native nsIDocumentPtr(nsIDocument);
 [ref] native nsIntRectRef(nsIntRect);
 [ptr] native nsIPresShellPtr(nsIPresShell);
 [ptr] native nsPresContextPtr(nsPresContext);
 [ptr] native nsViewPtr(nsView);
 [ptr] native nsDOMNavigationTimingPtr(nsDOMNavigationTiming);
 
-[scriptable, builtinclass, uuid(1b22be51-efe8-42ac-a9a0-06f50f39beee)]
+[scriptable, builtinclass, uuid(a73d693a-6260-468a-ae64-d64237f0858c)]
 interface nsIContentViewer : nsISupports
 {
 
   [noscript] void init(in nsIWidgetPtr aParentWidget,
                        [const] in nsIntRectRef aBounds);
 
   attribute nsIDocShell container;
 
@@ -54,16 +54,22 @@ interface nsIContentViewer : nsISupports
    * As above, but this passes around the aShouldPrompt argument to keep
    * track of whether the user has responded to a prompt.
    * Used internally by the scriptable version to ensure we only prompt once.
    */
   [noscript,nostdcall] boolean permitUnloadInternal(in boolean aCallerClosesWindow,
                                                     inout boolean aShouldPrompt);
 
   /**
+   * Exposes whether we're in the process of firing the beforeunload event.
+   * In this case, the corresponding docshell will not allow navigation.
+   */
+  readonly attribute boolean beforeUnloadFiring;
+
+  /**
    * Works in tandem with permitUnload, if the caller decides not to close the
    * window it indicated it will, it is the caller's responsibility to reset
    * that with this method.
    *
    * @Note this method is only meant to be called on documents for which the
    *  caller has indicated that it will close the window. If that is not the case
    *  the behavior of this method is undefined.
    */

--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -1226,16 +1226,23 @@ nsDocumentViewer::PermitUnloadInternal(b
 
   if (aCallerClosesWindow && *aPermitUnload)
     mCallerIsClosingWindow = true;
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
+nsDocumentViewer::GetBeforeUnloadFiring(bool* aInEvent)
+{
+  *aInEvent = mInPermitUnload;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
 nsDocumentViewer::ResetCloseWindow()
 {
   mCallerIsClosingWindow = false;
 
   nsCOMPtr<nsIDocShellTreeNode> docShellNode(mContainer);
   if (docShellNode) {
     int32_t childCount;
     docShellNode->GetChildCount(&childCount);