diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index a299bf980b70..83c6109b091c 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -4132,7 +4132,16 @@ nsDocShell::IsPrintingOrPP(bool aDisplayErrorDialog)
 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;
 }
 
 //*****************************************************************************
diff --git a/docshell/base/nsIContentViewer.idl b/docshell/base/nsIContentViewer.idl
index 38d642315dc3..f8d0a45bf2b5 100644
--- a/docshell/base/nsIContentViewer.idl
+++ b/docshell/base/nsIContentViewer.idl
@@ -28,7 +28,7 @@ class nsDOMNavigationTiming;
 [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
 {
 
@@ -58,6 +58,12 @@ interface nsIContentViewer : nsISupports
   [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
diff --git a/layout/base/nsDocumentViewer.cpp b/layout/base/nsDocumentViewer.cpp
index 51c7d6bfdab0..c88b6054bd5a 100644
--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -1239,6 +1239,13 @@ nsDocumentViewer::PermitUnloadInternal(bool aCallerClosesWindow,
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsDocumentViewer::GetBeforeUnloadFiring(bool* aInEvent)
+{
+  *aInEvent = mInPermitUnload;
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsDocumentViewer::ResetCloseWindow()
 {