--- a/content/base/src/nsGkAtomList.h
+++ b/content/base/src/nsGkAtomList.h
@@ -604,16 +604,17 @@ GK_ATOM(mousethrough, "mousethrough")
 GK_ATOM(mouseup, "mouseup")
 GK_ATOM(mozaudiochannel, "mozaudiochannel")
 GK_ATOM(mozasyncpanzoom, "mozasyncpanzoom")
 GK_ATOM(mozfullscreenchange, "mozfullscreenchange")
 GK_ATOM(mozfullscreenerror, "mozfullscreenerror")
 GK_ATOM(mozpasspointerevents, "mozpasspointerevents")
 GK_ATOM(mozpointerlockchange, "mozpointerlockchange")
 GK_ATOM(mozpointerlockerror, "mozpointerlockerror")
+GK_ATOM(mozprivatebrowsing, "mozprivatebrowsing")
 GK_ATOM(moz_opaque, "moz-opaque")
 GK_ATOM(moz_action_hint, "mozactionhint")
 GK_ATOM(x_moz_errormessage, "x-moz-errormessage")
 GK_ATOM(msthemecompatible, "msthemecompatible")
 GK_ATOM(multicol, "multicol")
 GK_ATOM(multiple, "multiple")
 GK_ATOM(muted, "muted")
 GK_ATOM(name, "name")

--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -846,16 +846,17 @@ nsDocShell::nsDocShell():
     mIsExecutingOnLoadHandler(false),
     mIsPrintingOrPP(false),
     mSavingOldViewer(false),
 #ifdef DEBUG
     mInEnsureScriptEnv(false),
 #endif
     mAffectPrivateSessionLifetime(true),
     mInvisible(false),
+    mHasLoadedNonBlankURI(false),
     mDefaultLoadFlags(nsIRequest::LOAD_NORMAL),
     mFrameType(eFrameTypeRegular),
     mOwnOrContainingAppId(nsIScriptSecurityManager::UNKNOWN_APP_ID),
     mParentCharsetSource(0)
 {
     mHistoryID = ++gDocshellIDCounter;
     if (gDocShellCount++ == 0) {
         NS_ASSERTION(sURIFixup == nullptr,
@@ -1923,16 +1924,20 @@ nsDocShell::SetCurrentURI(nsIURI *aURI, 
     // We don't want to send a location change when we're displaying an error
     // page, and we don't want to change our idea of "current URI" either
     if (mLoadType == LOAD_ERROR_PAGE) {
         return false;
     }
 
     mCurrentURI = NS_TryToMakeImmutable(aURI);
     
+    if (!NS_IsAboutBlank(mCurrentURI)) {
+      mHasLoadedNonBlankURI = true;
+    }
+
     bool isRoot = false;   // Is this the root docshell
     bool isSubFrame = false;  // Is this a subframe navigation?
 
     nsCOMPtr<nsIDocShellTreeItem> root;
 
     GetSameTypeRootTreeItem(getter_AddRefs(root));
     if (root.get() == static_cast<nsIDocShellTreeItem *>(this)) 
     {
@@ -2273,16 +2278,25 @@ nsDocShell::SetPrivateBrowsing(bool aUse
                 obs->PrivateModeChanged(aUsePrivateBrowsing);
             }
         }
     }
     return NS_OK;
 }
 
 NS_IMETHODIMP
+nsDocShell::GetHasLoadedNonBlankURI(bool* aResult)
+{
+    NS_ENSURE_ARG_POINTER(aResult);
+
+    *aResult = mHasLoadedNonBlankURI;
+    return NS_OK;
+}
+
+NS_IMETHODIMP
 nsDocShell::GetUseRemoteTabs(bool* aUseRemoteTabs)
 {
     NS_ENSURE_ARG_POINTER(aUseRemoteTabs);
 
     *aUseRemoteTabs = mUseRemoteTabs;
     return NS_OK;
 }
 

--- a/docshell/base/nsDocShell.h
+++ b/docshell/base/nsDocShell.h
@@ -910,16 +910,17 @@ protected:
     
     // @see nsIDocShellHistory::createdDynamically
     bool                       mDynamicallyCreated;
 #ifdef DEBUG
     bool                       mInEnsureScriptEnv;
 #endif
     bool                       mAffectPrivateSessionLifetime;
     bool                       mInvisible;
+    bool                       mHasLoadedNonBlankURI;
     uint64_t                   mHistoryID;
     uint32_t                   mDefaultLoadFlags;
 
     static nsIURIFixup *sURIFixup;
 
     nsRefPtr<nsDOMNavigationTiming> mTiming;
 
     // Are we a regular frame, a browser frame, or an app frame?

--- a/docshell/base/nsIDocShell.idl
+++ b/docshell/base/nsIDocShell.idl
@@ -49,17 +49,17 @@ interface nsIWebBrowserPrint;
 interface nsIVariant;
 interface nsIPrivacyTransitionObserver;
 interface nsIReflowObserver;
 interface nsIScrollObserver;
 interface nsITabParent;
  
 typedef unsigned long nsLoadFlags;
 
-[scriptable, builtinclass, uuid(2b8e4a50-7744-454d-a05b-debead8070fe)]
+[scriptable, builtinclass, uuid(23157a63-26fd-44a0-a0f9-fdc64dcc004c)]
 interface nsIDocShell : nsIDocShellTreeItem
 {
   /**
    * Loads a given URI.  This will give priority to loading the requested URI
    * in the object implementing	this interface.  If it can't be loaded here
    * however, the URL dispatcher will go through its normal process of content
    * loading.
    *
@@ -1012,9 +1012,16 @@ interface nsIDocShell : nsIDocShellTreeI
    * See the documentation for setOpener and getOpener about why we
    * don't use attribute here instead.
    */
   [noscript,notxpcom,nostdcall] void setOpenedRemote(in nsITabParent aOpenedRemote);
   [noscript,notxpcom,nostdcall] nsITabParent getOpenedRemote();
 
   // URLSearchParams for the window.location is owned by the docShell.
   [noscript,notxpcom] URLSearchParams getURLSearchParams();
+
+  /**
+   * This attribute determines whether a document which is not about:blank has
+   * already be loaded by this docShell.
+   */
+  [infallible] readonly attribute boolean hasLoadedNonBlankURI;
+
 };

--- a/dom/browser-element/BrowserElementChild.js
+++ b/dom/browser-element/BrowserElementChild.js
@@ -51,8 +51,15 @@ if (!('BrowserElementIsPreloaded' in thi
 }
 
 var BrowserElementIsReady = true;
 
 let infos = sendSyncMessage('browser-element-api:call',
                             { 'msg_name': 'hello' })[0];
 docShell.QueryInterface(Ci.nsIDocShellTreeItem).name = infos.name;
 docShell.setFullscreenAllowed(infos.fullscreenAllowed);
+if (infos.isPrivate) {
+  if (!docShell.hasLoadedNonBlankURI) {
+    Cu.reportError("We should not switch to Private Browsing after loading a document.");
+  } else {
+    docShell.QueryInterface(Ci.nsILoadContext).usePrivateBrowsing = true;
+  }
+}

--- a/dom/browser-element/BrowserElementParent.cpp
+++ b/dom/browser-element/BrowserElementParent.cpp
@@ -71,16 +71,24 @@ CreateIframe(Element* aOpenerFrameElemen
                              aName, /* aNotify = */ false);
 
   // Indicate whether the iframe is should be remote.
   popupFrameElement->SetAttr(kNameSpaceID_None, nsGkAtoms::Remote,
                              aRemote ? NS_LITERAL_STRING("true") :
                                        NS_LITERAL_STRING("false"),
                              /* aNotify = */ false);
 
+  // Copy the opener frame's mozprivatebrowsing attribute to the popup frame.
+  nsAutoString mozprivatebrowsing;
+  if (aOpenerFrameElement->GetAttr(kNameSpaceID_None, nsGkAtoms::mozprivatebrowsing,
+                                   mozprivatebrowsing)) {
+    popupFrameElement->SetAttr(kNameSpaceID_None, nsGkAtoms::mozprivatebrowsing,
+                               mozprivatebrowsing, /* aNotify = */ false);
+  }
+
   return popupFrameElement.forget();
 }
 
 bool
 DispatchCustomDOMEvent(Element* aFrameElement, const nsAString& aEventName,
                        JSContext* cx, JS::Handle<JS::Value> aDetailValue,
                        nsEventStatus *aStatus)
 {

--- a/dom/browser-element/BrowserElementParent.jsm
+++ b/dom/browser-element/BrowserElementParent.jsm
@@ -384,17 +384,18 @@ BrowserElementParent.prototype = {
       this._domRequestReady = true;
       this._runPendingAPICall();
     }
 
     return {
       name: this._frameElement.getAttribute('name'),
       fullscreenAllowed:
         this._frameElement.hasAttribute('allowfullscreen') ||
-        this._frameElement.hasAttribute('mozallowfullscreen')
+        this._frameElement.hasAttribute('mozallowfullscreen'),
+      isPrivate: this._frameElement.hasAttribute('mozprivatebrowsing')
     };
   },
 
   _fireCtxMenuEvent: function(data) {
     let detail = data.json;
     let evtName = detail.msg_name;
 
     debug('fireCtxMenuEventFromMsg: ' + evtName + ' ' + detail);