diff --git a/modules/plugin/base/public/npapi.h b/modules/plugin/base/public/npapi.h
index 80b1f24da4e..394bd40b9f7 100644
--- a/modules/plugin/base/public/npapi.h
+++ b/modules/plugin/base/public/npapi.h
@@ -91,7 +91,7 @@
 /*----------------------------------------------------------------------*/
 
 #define NP_VERSION_MAJOR 0
-#define NP_VERSION_MINOR 21
+#define NP_VERSION_MINOR 22
 
 
 /* The OS/2 version of Netscape uses RC_DATA to define the
@@ -327,7 +327,10 @@ typedef enum {
   /* Checks if the plugin is interested in receiving the http body of
    * all http requests (including failed ones, http status != 200).
    */
-  NPPVpluginWantsAllNetworkStreams = 18
+  NPPVpluginWantsAllNetworkStreams = 18,
+
+  NPPVprivateModeBool = 19
+
 #ifdef XP_MACOSX
   /* Used for negotiating drawing models */
   , NPPVpluginDrawingModel = 1000
@@ -358,7 +361,9 @@ typedef enum {
   /* Get the NPObject wrapper for the plugins DOM element. */
   NPNVPluginElementNPObject = 16,
 
-  NPNVSupportsWindowless = 17
+  NPNVSupportsWindowless = 17,
+
+  NPNVprivateModeBool = 18
 
 #ifdef XP_MACOSX
   /* Used for negotiating drawing models */
diff --git a/modules/plugin/base/public/npfunctions.h b/modules/plugin/base/public/npfunctions.h
index f3b5fdd3d5b..c6caaf0b509 100644
--- a/modules/plugin/base/public/npfunctions.h
+++ b/modules/plugin/base/public/npfunctions.h
@@ -61,10 +61,10 @@ typedef void         (*NPP_URLNotifyProcPtr)(NPP instance, const char* url, NPRe
 /* Any NPObjects returned to the browser via NPP_GetValue should be retained
    by the plugin on the way out. The browser is responsible for releasing. */
 typedef NPError      (*NPP_GetValueProcPtr)(NPP instance, NPPVariable variable, void *ret_value);
-typedef NPError      (*NPP_SetValueProcPtr)(NPP instance, NPNVariable variable, void *ret_value);
+typedef NPError      (*NPP_SetValueProcPtr)(NPP instance, NPNVariable variable, void *value);
 
 typedef NPError      (*NPN_GetValueProcPtr)(NPP instance, NPNVariable variable, void *ret_value);
-typedef NPError      (*NPN_SetValueProcPtr)(NPP instance, NPPVariable variable, void *ret_value);
+typedef NPError      (*NPN_SetValueProcPtr)(NPP instance, NPPVariable variable, void *value);
 typedef NPError      (*NPN_GetURLNotifyProcPtr)(NPP instance, const char* url, const char* window, void* notifyData);
 typedef NPError      (*NPN_PostURLNotifyProcPtr)(NPP instance, const char* url, const char* window, uint32_t len, const char* buf, NPBool file, void* notifyData);
 typedef NPError      (*NPN_GetURLProcPtr)(NPP instance, const char* url, const char* window);
diff --git a/modules/plugin/base/src/nsNPAPIPlugin.cpp b/modules/plugin/base/src/nsNPAPIPlugin.cpp
index 5b15f8d9cf0..c313ccac87b 100644
--- a/modules/plugin/base/src/nsNPAPIPlugin.cpp
+++ b/modules/plugin/base/src/nsNPAPIPlugin.cpp
@@ -45,6 +45,7 @@
 #include "nsNPAPIPluginStreamListener.h"
 #include "nsIServiceManager.h"
 #include "nsThreadUtils.h"
+#include "nsIPrivateBrowsingService.h"
 
 #include "nsIPluginStreamListener.h"
 #include "nsPluginsDir.h"
@@ -2042,6 +2043,15 @@ _getvalue(NPP npp, NPNVariable variable, void *result)
     return NPERR_NO_ERROR;
   }
 
+  case NPNVprivateModeBool: {
+    nsCOMPtr<nsIPrivateBrowsingService> pbs = do_GetService(NS_PRIVATE_BROWSING_SERVICE_CONTRACTID);
+    if (pbs) {
+      pbs->GetPrivateBrowsingEnabled((PRBool*)result);
+      return NPERR_NO_ERROR;
+    }
+    return NPERR_GENERIC_ERROR;
+  }
+
 #ifdef XP_MACOSX
   case NPNVpluginDrawingModel: {
     if (npp) {
diff --git a/modules/plugin/base/src/nsNPAPIPluginInstance.cpp b/modules/plugin/base/src/nsNPAPIPluginInstance.cpp
index ab20a9ddbf6..b2fe9e75d6c 100644
--- a/modules/plugin/base/src/nsNPAPIPluginInstance.cpp
+++ b/modules/plugin/base/src/nsNPAPIPluginInstance.cpp
@@ -48,6 +48,7 @@
 #include "nsPluginHostImpl.h"
 #include "nsPluginSafety.h"
 #include "nsPluginLogging.h"
+#include "nsIPrivateBrowsingService.h"
 
 #include "nsPIPluginInstancePeer.h"
 #include "nsPIDOMWindow.h"
@@ -1482,3 +1483,28 @@ nsNPAPIPluginInstance::GetPluginAPIVersion()
 {
   return fCallbacks->version;
 }
+
+nsresult nsNPAPIPluginInstance::PrivateModeStateChanged()
+{
+  if (!mStarted)
+    return NS_OK;
+  
+  PLUGIN_LOG(PLUGIN_LOG_NORMAL, ("nsNPAPIPluginInstance informing plugin of private mode state change this=%p\n",this));
+  
+  if (fCallbacks->setvalue) {
+    PluginDestructionGuard guard(this);
+    
+    nsCOMPtr<nsIPrivateBrowsingService> pbs = do_GetService(NS_PRIVATE_BROWSING_SERVICE_CONTRACTID);
+    if (pbs) {
+      PRBool pme = PR_FALSE;
+      nsresult rv = pbs->GetPrivateBrowsingEnabled(&pme);
+      if (NS_FAILED(rv))
+        return rv;
+
+      NPError error;
+      NS_TRY_SAFE_CALL_RETURN(error, (*fCallbacks->setvalue)(&fNPP, NPNVprivateModeBool, &pme), fLibrary, this);
+      return (error == NPERR_NO_ERROR) ? NS_OK : NS_ERROR_FAILURE;
+    }
+  }
+  return NS_ERROR_FAILURE;
+}
diff --git a/modules/plugin/base/src/nsNPAPIPluginInstance.h b/modules/plugin/base/src/nsNPAPIPluginInstance.h
index 950006c93c3..45f7325dda0 100644
--- a/modules/plugin/base/src/nsNPAPIPluginInstance.h
+++ b/modules/plugin/base/src/nsNPAPIPluginInstance.h
@@ -89,7 +89,6 @@ public:
 
     // nsNPAPIPluginInstance-specific methods
 
-    // Return the 4.x-style interface object.
     nsresult GetNPP(NPP * aNPP);
 
     // Return the callbacks for the plugin instance.
@@ -132,6 +131,7 @@ public:
 
     already_AddRefed<nsPIDOMWindow> GetDOMWindow();
 
+    nsresult PrivateModeStateChanged();
 protected:
 
     nsresult InitializePlugin(nsIPluginInstancePeer* peer);
diff --git a/modules/plugin/base/src/nsPluginHostImpl.cpp b/modules/plugin/base/src/nsPluginHostImpl.cpp
index 8857b626fc3..df675cfe61d 100644
--- a/modules/plugin/base/src/nsPluginHostImpl.cpp
+++ b/modules/plugin/base/src/nsPluginHostImpl.cpp
@@ -94,6 +94,7 @@
 #include "nsPrintfCString.h"
 #include "nsIBlocklistService.h"
 #include "nsVersionComparator.h"
+#include "nsIPrivateBrowsingService.h"
 
 // Friggin' X11 has to "#define None". Lame!
 #ifdef None
@@ -1071,7 +1072,7 @@ nsresult PostPluginUnloadEvent(PRLibrary* aLibrary)
 void nsPluginTag::TryUnloadPlugin(PRBool aForceShutdown)
 {
   PRBool isXPCOM = PR_FALSE;
-  if (!(mFlags & NS_PLUGIN_FLAG_OLDSCHOOL))
+  if (!(mFlags & NS_PLUGIN_FLAG_NPAPI))
     isXPCOM = PR_TRUE;
 
   if (isXPCOM && !aForceShutdown) return;
@@ -2519,9 +2520,9 @@ nsPluginHostImpl::nsPluginHostImpl()
   }
 
   nsCOMPtr<nsIObserverService> obsService = do_GetService("@mozilla.org/observer-service;1");
-  if (obsService)
-  {
+  if (obsService) {
     obsService->AddObserver(this, NS_XPCOM_SHUTDOWN_OBSERVER_ID, PR_FALSE);
+    obsService->AddObserver(this, NS_PRIVATE_BROWSING_SWITCH_TOPIC, PR_FALSE);
   }
 
 #ifdef PLUGIN_LOGGING
@@ -2688,7 +2689,7 @@ nsresult nsPluginHostImpl::ReloadPlugins(PRBool reloadPages)
     // if plugins are reloaded. This also fixes a crash on UNIX where the call
     // to shutdown would break the ProxyJNI connection to the JRE after a reload.
     // see bug 86591
-    if (!IsRunningPlugin(p) && (!p->mEntryPoint || p->HasFlag(NS_PLUGIN_FLAG_OLDSCHOOL))) {
+    if (!IsRunningPlugin(p) && (!p->mEntryPoint || p->HasFlag(NS_PLUGIN_FLAG_NPAPI))) {
       if (p == mPlugins)
         mPlugins = next;
       else
@@ -4529,11 +4530,11 @@ NS_IMETHODIMP nsPluginHostImpl::GetPluginFactory(const char *aMimeType, nsIPlugi
       }
 #endif
       else {
-        // Now lets try to get the entry point from a 4.x plugin
+        // Now lets try to get the entry point from an NPAPI plugin
         rv = CreateNPAPIPlugin(serviceManager, pluginTag, &plugin);
         if (NS_SUCCEEDED(rv))
           pluginTag->mEntryPoint = plugin;
-        pluginTag->Mark(NS_PLUGIN_FLAG_OLDSCHOOL);
+        pluginTag->Mark(NS_PLUGIN_FLAG_NPAPI);
         // no need to initialize, already done by CreatePlugin()
       }
     }
@@ -6142,6 +6143,16 @@ NS_IMETHODIMP nsPluginHostImpl::Observe(nsISupports *aSubject,
     UnloadUnusedLibraries();
     sInst->Release();
   }
+  if (!nsCRT::strcmp(NS_PRIVATE_BROWSING_SWITCH_TOPIC, aTopic)) {
+    // inform all active NPAPI plugins of changed private mode state
+    for (nsActivePlugin* ap = mActivePluginList.mFirst; ap; ap = ap->mNext) {
+      nsPluginTag* pt = ap->mPluginTag;
+      if (pt->HasFlag(NS_PLUGIN_FLAG_NPAPI)) {
+        nsNPAPIPluginInstance* pi = static_cast<nsNPAPIPluginInstance*>(ap->mInstance);
+        pi->PrivateModeStateChanged();
+      }
+    }
+  }
   if (!nsCRT::strcmp(NS_PREFBRANCH_PREFCHANGE_TOPIC_ID, aTopic)) {
     NS_ASSERTION(someData &&
                  nsDependentString(someData).EqualsLiteral("security.enable_java"),
@@ -6161,7 +6172,7 @@ NS_IMETHODIMP nsPluginHostImpl::Observe(nsISupports *aSubject,
       for (nsPluginTag* cur = mPlugins; cur; cur = cur->mNext) {
         if (cur->mIsJavaPlugin)
           cur->SetDisabled(!mJavaEnabled);
-      }            
+      }
     }
   }
   return NS_OK;
@@ -6297,7 +6308,6 @@ nsPluginHostImpl::ParsePostBufferToFixHeaders(
   const char *pEoh = 0;   // pointer to end of headers in inPostData
   const char *pEod = inPostData + inPostDataLen; // pointer to end of inPostData
   if (*inPostData == LF) {
-    // from 4.x spec http://developer.netscape.com/docs/manuals/communicator/plugin/pgfn2.htm#1007754
     // If no custom headers are required, simply add a blank
     // line ('\n') to the beginning of the file or buffer.
     // so *inPostData == '\n' is valid
@@ -6500,8 +6510,6 @@ nsPluginHostImpl::CreateTmpFileToPost(const char *postDataURL, char **pTmpFileNa
       if (NS_FAILED(rv) || (PRInt32)br <= 0)
         break;
       if (firstRead) {
-        // according to the 4.x spec
-        // http://developer.netscape.com/docs/manuals/communicator/plugin/pgfn2.htm#1007707
         //"For protocols in which the headers must be distinguished from the body,
         // such as HTTP, the buffer or file should contain the headers, followed by
         // a blank line, then the body. If no custom headers are required, simply
diff --git a/modules/plugin/base/src/nsPluginHostImpl.h b/modules/plugin/base/src/nsPluginHostImpl.h
index e7686b34030..82ca9001f72 100644
--- a/modules/plugin/base/src/nsPluginHostImpl.h
+++ b/modules/plugin/base/src/nsPluginHostImpl.h
@@ -79,7 +79,7 @@ class nsIRegistry;
 class nsPluginHostImpl;
 
 #define NS_PLUGIN_FLAG_ENABLED      0x0001    // is this plugin enabled?
-#define NS_PLUGIN_FLAG_OLDSCHOOL    0x0002    // is this a pre-xpcom plugin?
+#define NS_PLUGIN_FLAG_NPAPI        0x0002    // is this an NPAPI plugin?
 #define NS_PLUGIN_FLAG_FROMCACHE    0x0004    // this plugintag info was loaded from cache
 #define NS_PLUGIN_FLAG_UNWANTED     0x0008    // this is an unwanted plugin
 #define NS_PLUGIN_FLAG_BLOCKLISTED  0x0010    // this is a blocklisted plugin
diff --git a/modules/plugin/test/mochitest/Makefile.in b/modules/plugin/test/mochitest/Makefile.in
index 72ec45cc89f..b768e92144d 100644
--- a/modules/plugin/test/mochitest/Makefile.in
+++ b/modules/plugin/test/mochitest/Makefile.in
@@ -45,6 +45,10 @@ include $(DEPTH)/config/autoconf.mk
 include $(topsrcdir)/config/rules.mk
 
 _TEST_FILES =	test_npruntime.xul
+_TEST_FILES = \
+		test_npruntime.xul   \
+    test_privatemode.xul \
+		$(NULL)
 
 libs:: $(_TEST_FILES)
 	$(INSTALL) $^ $(DEPTH)/_tests/testing/mochitest/chrome/$(relativesrcdir)
diff --git a/modules/plugin/test/mochitest/test_privatemode.xul b/modules/plugin/test/mochitest/test_privatemode.xul
new file mode 100644
index 00000000000..31b6893f111
--- /dev/null
+++ b/modules/plugin/test/mochitest/test_privatemode.xul
@@ -0,0 +1,69 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin" type="text/css"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css"
+                 type="text/css"?>
+<window title="NPAPI Private Mode Tests"
+  xmlns:html="http://www.w3.org/1999/xhtml"
+  xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <title>NPAPI Private Mode Tests</title>
+  <script type="application/javascript" 
+   src="chrome://mochikit/content/MochiKit/packed.js"></script>
+  <script type="application/javascript"
+          src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js" />
+<body xmlns="http://www.w3.org/1999/xhtml" onload="runTests()">
+<embed id="plugin1" type="application/x-test" width="200" height="200"></embed>
+<embed id="plugin2" type="application/x-test" width="200" height="200"></embed>
+</body>
+<script class="testbody" type="application/javascript">
+<![CDATA[
+SimpleTest.waitForExplicitFinish();
+
+function runTests() {
+  var pluginElement1 = document.getElementById("plugin1");
+  var pluginElement2 = document.getElementById("plugin2");
+
+  var state1 = false;
+  var state2 = false;
+  var exceptionThrown = false;
+
+  try {
+    state1 = pluginElement1.queryPrivateModeState();
+    state2 = pluginElement2.queryPrivateModeState();
+  } catch (e) {
+    exceptionThrown = true;
+  }
+  is(exceptionThrown, false, "Exception thrown getting private mode state.");
+  is(state1, false, "Browser returned incorrect private mode state.");
+  is(state2, false, "Browser returned incorrect private mode state.");
+
+  // change private mode pref
+  var prefs = Components.classes["@mozilla.org/preferences-service;1"].getService(Components.interfaces.nsIPrefBranch);
+  var privateBrowsing = Components.classes["@mozilla.org/privatebrowsing;1"].getService(Components.interfaces.nsIPrivateBrowsingService);
+  var keepCurrentSession;
+  try {
+    keepCurrentSession = prefs.getBoolPref("browser.privatebrowsing.keep_current_session");
+  } catch (e) {
+    keepCurrentSession = false
+  }
+  prefs.setBoolPref("browser.privatebrowsing.keep_current_session", true);
+  privateBrowsing.privateBrowsingEnabled = true;
+
+  try {
+    state1 = pluginElement1.lastReportedPrivateModeState();
+    state2 = pluginElement2.lastReportedPrivateModeState();
+  } catch (e) {
+    exceptionThrown = true;
+  }
+  is(exceptionThrown, false, "Exception thrown getting private mode state.");
+  is(state1, true, "Private mode state reported incorrectly.");
+  is(state2, true, "Private mode state reported incorrectly.");
+
+  // reset preference states
+  privateBrowsing.privateBrowsingEnabled = false;
+  prefs.setBoolPref("browser.privatebrowsing.keep_current_session", keepCurrentSession);
+
+  SimpleTest.finish();
+}
+]]>
+</script>
+</window>
diff --git a/modules/plugin/test/testplugin/nptest.cpp b/modules/plugin/test/testplugin/nptest.cpp
index 0cc914a4ab8..8a88298e794 100644
--- a/modules/plugin/test/testplugin/nptest.cpp
+++ b/modules/plugin/test/testplugin/nptest.cpp
@@ -55,12 +55,16 @@ static NPClass sNPClass;
 // identifiers
 //
 
-#define IDENTIFIER_TO_STRING_TEST_METHOD 0
-#define NUM_METHOD_IDENTIFIERS           1
+#define IDENTIFIER_TO_STRING_TEST_METHOD        0
+#define QUERY_PRIVATE_MODE_STATE_METHOD         1
+#define LAST_REPORTED_PRIVATE_MODE_STATE_METHOD 2
+#define NUM_METHOD_IDENTIFIERS                  3
 
 static NPIdentifier sPluginMethodIdentifiers[NUM_METHOD_IDENTIFIERS];
 static const NPUTF8 *sPluginMethodIdentifierNames[NUM_METHOD_IDENTIFIERS] = {
   "identifierToStringTest",
+  "queryPrivateModeState",
+  "lastReportedPrivateModeState",
 };
 
 static bool sIdentifiersInitialized = false;
@@ -83,7 +87,9 @@ static void clearIdentifiers()
 // function signatures
 //
 
-bool identifierToStringTest(const NPVariant* args, uint32_t argCount, NPVariant* result);
+bool identifierToStringTest(NPObject* npobj, const NPVariant* args, uint32_t argCount, NPVariant* result);
+bool queryPrivateModeState(NPObject* npobj, const NPVariant* args, uint32_t argCount, NPVariant* result);
+bool lastReportedPrivateModeState(NPObject* npobj, const NPVariant* args, uint32_t argCount, NPVariant* result);
 
 NPObject* scriptableAllocate(NPP npp, NPClass* aClass);
 void scriptableDeallocate(NPObject* npobj);
@@ -249,6 +255,8 @@ NPP_New(NPMIMEType pluginType, NPP instance, uint16_t mode, int16_t argc, char*
     }
   }
 
+  instanceData->lastReportedPrivateModeState = false;
+
   // do platform-specific initialization
   NPError err = pluginInstanceInit(instanceData);
   if (err != NPERR_NO_ERROR)
@@ -337,6 +345,11 @@ NPP_GetValue(NPP instance, NPPVariable variable, void* value)
 NPError
 NPP_SetValue(NPP instance, NPNVariable variable, void* value)
 {
+  if (variable == NPNVprivateModeBool) {
+    InstanceData* instanceData = (InstanceData*)(instance->pdata);
+    instanceData->lastReportedPrivateModeState = *static_cast<NPBool*>(value);
+    return NPERR_NO_ERROR;
+  }
   return NPERR_GENERIC_ERROR;
 }
 
@@ -473,7 +486,11 @@ bool
 scriptableInvoke(NPObject* npobj, NPIdentifier name, const NPVariant* args, uint32_t argCount, NPVariant* result)
 {
   if (name == sPluginMethodIdentifiers[IDENTIFIER_TO_STRING_TEST_METHOD])
-    return identifierToStringTest(args, argCount, result);
+    return identifierToStringTest(npobj, args, argCount, result);
+  else if (name == sPluginMethodIdentifiers[QUERY_PRIVATE_MODE_STATE_METHOD])
+    return queryPrivateModeState(npobj, args, argCount, result);
+  else if (name == sPluginMethodIdentifiers[LAST_REPORTED_PRIVATE_MODE_STATE_METHOD])
+    return lastReportedPrivateModeState(npobj, args, argCount, result);
   return false;
 }
 
@@ -524,7 +541,7 @@ scriptableConstruct(NPObject* npobj, const NPVariant* args, uint32_t argCount, N
 //
 
 bool
-identifierToStringTest(const NPVariant* args, uint32_t argCount, NPVariant* result)
+identifierToStringTest(NPObject* npobj, const NPVariant* args, uint32_t argCount, NPVariant* result)
 {
   if (argCount != 1)
     return false;
@@ -537,3 +554,21 @@ identifierToStringTest(const NPVariant* args, uint32_t argCount, NPVariant* resu
   STRINGZ_TO_NPVARIANT(utf8String, *result);
   return true;
 }
+
+bool
+queryPrivateModeState(NPObject* npobj, const NPVariant* args, uint32_t argCount, NPVariant* result)
+{
+  NPBool pms = false;
+  NPN_GetValue(static_cast<TestNPObject*>(npobj)->npp, NPNVprivateModeBool, &pms);
+  BOOLEAN_TO_NPVARIANT(pms, *result);
+  return true;
+}
+
+bool
+lastReportedPrivateModeState(NPObject* npobj, const NPVariant* args, uint32_t argCount, NPVariant* result)
+{
+  InstanceData* id = static_cast<InstanceData*>(static_cast<TestNPObject*>(npobj)->npp->pdata);
+  NPBool pms = id->lastReportedPrivateModeState;
+  BOOLEAN_TO_NPVARIANT(pms, *result);
+  return true;
+}
diff --git a/modules/plugin/test/testplugin/nptest.h b/modules/plugin/test/testplugin/nptest.h
index ced3a1267d4..32d075ceee9 100644
--- a/modules/plugin/test/testplugin/nptest.h
+++ b/modules/plugin/test/testplugin/nptest.h
@@ -49,14 +49,14 @@ typedef enum  {
 typedef struct TestNPObject : NPObject {
   NPP npp;
   DrawMode drawMode;
-  // 0xAARRGGBB
-  PRUint32 drawColor;
+  PRUint32 drawColor; // 0xAARRGGBB
 } TestNPObject;
 
 typedef struct InstanceData {
   NPP npp;
   NPWindow window;
   TestNPObject* scriptableObject;
+  NPBool lastReportedPrivateModeState;
 } InstanceData;
 
 #endif // nptest_h_