diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index 7faa360321a..6464e5cdcf5 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -242,6 +242,9 @@ static PRInt32 gNumberOfDocumentsLoading = 0;
 // Global count of existing docshells.
 static PRInt32 gDocShellCount = 0;
 
+// Global count of docshells with the private attribute set
+static PRUint32 gNumberOfPrivateDocShells = 0;
+
 // Global reference to the URI fixup service.
 nsIURIFixup *nsDocShell::sURIFixup = 0;
 
@@ -710,6 +713,19 @@ ConvertLoadTypeToNavigationType(PRUint32 aLoadType)
 
 static nsISHEntry* GetRootSHEntry(nsISHEntry *entry);
 
+static void
+DecreasePrivateDocShellCount()
+{
+    MOZ_ASSERT(gNumberOfPrivateDocShells > 0);
+    gNumberOfPrivateDocShells--;
+    if (!gNumberOfPrivateDocShells)
+    {
+        nsCOMPtr<nsIObserverService> obsvc = mozilla::services::GetObserverService();
+        if (obsvc)
+            obsvc->NotifyObservers(nsnull, "last-pb-context-exited", nsnull);
+    }
+}
+
 //*****************************************************************************
 //***    nsDocShell: Object Management
 //*****************************************************************************
@@ -816,6 +832,10 @@ nsDocShell::~nsDocShell()
         printf("--DOCSHELL %p == %ld\n", (void*) this, gNumberOfDocShells);
     }
 #endif
+
+    if (mInPrivateBrowsing) {
+        DecreasePrivateDocShellCount();
+    }
 }
 
 nsresult
@@ -1998,7 +2018,12 @@ NS_IMETHODIMP
 nsDocShell::SetUsePrivateBrowsing(bool aUsePrivateBrowsing)
 {
     mInPrivateBrowsing = aUsePrivateBrowsing;
-
+    if (aUsePrivateBrowsing) {
+        gNumberOfPrivateDocShells++;
+    } else {
+        DecreasePrivateDocShellCount();
+    }
+    
     PRInt32 count = mChildList.Count();
     for (PRInt32 i = 0; i < count; ++i) {
         nsCOMPtr<nsILoadContext> shell = do_QueryInterface(ChildAt(i));