--- a/b2g/app/nsBrowserApp.cpp
+++ b/b2g/app/nsBrowserApp.cpp
@@ -140,22 +140,22 @@ static int do_main(int argc, char* argv[
 
   if (appini) {
     nsXREAppData *appData;
     rv = XRE_CreateAppData(appini, &appData);
     if (NS_FAILED(rv)) {
       Output("Couldn't read application.ini");
       return 255;
     }
-    int result = XRE_main(argc, argv, appData);
+    int result = XRE_main(argc, argv, appData, 0);
     XRE_FreeAppData(appData);
     return result;
   }
 
-  return XRE_main(argc, argv, &sAppData);
+  return XRE_main(argc, argv, &sAppData, 0);
 }
 
 int main(int argc, char* argv[])
 {
   char exePath[MAXPATHLEN];
 
   nsresult rv = mozilla::BinaryPath::Get(argv[0], exePath);
   if (NS_FAILED(rv)) {

--- a/browser/app/nsBrowserApp.cpp
+++ b/browser/app/nsBrowserApp.cpp
@@ -144,22 +144,22 @@ static int do_main(int argc, char* argv[
 
   if (appini) {
     nsXREAppData *appData;
     rv = XRE_CreateAppData(appini, &appData);
     if (NS_FAILED(rv)) {
       Output("Couldn't read application.ini");
       return 255;
     }
-    int result = XRE_main(argc, argv, appData);
+    int result = XRE_main(argc, argv, appData, 0);
     XRE_FreeAppData(appData);
     return result;
   }
 
-  return XRE_main(argc, argv, &sAppData);
+  return XRE_main(argc, argv, &sAppData, 0);
 }
 
 int main(int argc, char* argv[])
 {
   char exePath[MAXPATHLEN];
 
 #ifdef XP_MACOSX
   TriggerQuirks();

--- a/mobile/xul/app/nsBrowserApp.cpp
+++ b/mobile/xul/app/nsBrowserApp.cpp
@@ -147,22 +147,22 @@ static int do_main(int argc, char* argv[
 
   if (appini) {
     nsXREAppData *appData;
     rv = XRE_CreateAppData(appini, &appData);
     if (NS_FAILED(rv)) {
       Output("Couldn't read application.ini");
       return 255;
     }
-    int result = XRE_main(argc, argv, appData);
+    int result = XRE_main(argc, argv, appData, 0);
     XRE_FreeAppData(appData);
     return result;
   }
 
-  return XRE_main(argc, argv, &sAppData);
+  return XRE_main(argc, argv, &sAppData, 0);
 }
 
 #if MOZ_PLATFORM_MAEMO == 6
 static bool
 GeckoPreLoader(const char* execPath)
 {
   nsresult rv = XPCOMGlueStartup(execPath);
   if (NS_FAILED(rv)) {

--- a/toolkit/library/dlldeps-xul.cpp
+++ b/toolkit/library/dlldeps-xul.cpp
@@ -1,17 +1,17 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "nsXULAppAPI.h"
 
 void xxxNeverCalledXUL()
 {
-  XRE_main(0, nsnull, nsnull);
+  XRE_main(0, nsnull, nsnull, nsnull);
   XRE_GetFileFromPath(nsnull, nsnull);
   XRE_LockProfileDirectory(nsnull, nsnull);
   XRE_InitEmbedding2(nsnull, nsnull, nsnull);
   XRE_NotifyProfile();
   XRE_TermEmbedding();
   XRE_CreateAppData(nsnull, nsnull);
   XRE_ParseAppData(nsnull, nsnull);
   XRE_FreeAppData(nsnull);

--- a/toolkit/xre/nsAndroidStartup.cpp
+++ b/toolkit/xre/nsAndroidStartup.cpp
@@ -65,17 +65,17 @@ GeckoStart(void *data, const nsXREAppDat
     nsTArray<char *> targs;
     char *arg = strtok(static_cast<char *>(data), " ");
     while (arg) {
         targs.AppendElement(arg);
         arg = strtok(NULL, " ");
     }
     targs.AppendElement(static_cast<char *>(nsnull));
 
-    int result = XRE_main(targs.Length() - 1, targs.Elements(), appData);
+    int result = XRE_main(targs.Length() - 1, targs.Elements(), appData, 0);
 
     if (result)
         LOG("XRE_main returned %d", result);
 
     mozilla::AndroidBridge::Bridge()->NotifyXreExit();
 
     free(targs[0]);
     nsMemory::Free(data);

--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -3907,17 +3907,17 @@ XREMain::XRE_main(int argc, char* argv[]
 #endif
 
   XRE_DeinitCommandLine();
 
   return NS_FAILED(rv) ? 1 : 0;
 }
 
 int
-XRE_main(int argc, char* argv[], const nsXREAppData* aAppData)
+XRE_main(int argc, char* argv[], const nsXREAppData* aAppData, PRUint32 aFlags)
 {
   XREMain main;
   return main.XRE_main(argc, argv, aAppData);
 }
 
 nsresult
 XRE_InitCommandLine(int aArgc, char* aArgv[])
 {

--- a/toolkit/xre/nsEmbedFunctions.cpp
+++ b/toolkit/xre/nsEmbedFunctions.cpp
@@ -741,8 +741,26 @@ XRE_ShutdownTestShell()
 
 #ifdef MOZ_X11
 void
 XRE_InstallX11ErrorHandler()
 {
   InstallX11ErrorHandler();
 }
 #endif
+
+#ifdef XP_WIN
+static WindowsEnvironmentType
+sWindowsEnvironmentType = WindowsEnvironmentType_Desktop;
+
+void
+SetWindowsEnvironment(WindowsEnvironmentType aEnvID)
+{
+  sWindowsEnvironmentType = aEnvID;
+}
+
+WindowsEnvironmentType
+XRE_GetWindowsEnvironment()
+{
+  return sWindowsEnvironmentType;
+}
+#endif // XP_WIN
+

--- a/webapprt/mac/webapprt.mm
+++ b/webapprt/mac/webapprt.mm
@@ -266,17 +266,17 @@ main(int argc, char **argv)
           @throw MakeException(@"Error", @"Unable to open application XRE directory.");
         }
 
         xreDir.forget(&webShellAppData->xreDirectory);
         NS_IF_RELEASE(webShellAppData->directory);
         directory.forget(&webShellAppData->directory);
 
         // There is only XUL.
-        result = XRE_main(argc, argv, webShellAppData);
+        result = XRE_main(argc, argv, webShellAppData, 0);
 
         XRE_FreeAppData(webShellAppData);
       }
       NS_LogTerm();
 
       return result;
     }
 

--- a/webapprt/win/webapprt.cpp
+++ b/webapprt/win/webapprt.cpp
@@ -297,17 +297,17 @@ namespace {
       rv = XRE_GetFileFromPath(greDir, getter_AddRefs(xreDir));
       NS_ENSURE_SUCCESS(rv, false);
 
       xreDir.forget(&webShellAppData->xreDirectory);
       NS_IF_RELEASE(webShellAppData->directory);
       directory.forget(&webShellAppData->directory);
 
       // There is only XUL.
-      XRE_main(*pargc, *pargv, webShellAppData);
+      XRE_main(*pargc, *pargv, webShellAppData, 0);
     }
 
     return true;
   }
 
   bool
   AttemptLoadFromDir(char* firefoxDir)
   {

--- a/xpcom/build/nsXULAppAPI.h
+++ b/xpcom/build/nsXULAppAPI.h
@@ -120,33 +120,43 @@
  * Windows: Documents and Settings\<User>\Local Settings\Application Data\
  *          <Vendor>\<Application>\<relative path to app dir from Program Files>
  * If appDir is not under the Program Files, directory service will fail.
  * Callers should fallback to appDir.
  */
 #define XRE_UPDATE_ROOT_DIR "UpdRootD"
 
 /**
+ * Platform flag values for XRE_main.
+ *
+ * XRE_MAIN_FLAG_USE_METRO - On Windows, use the winrt backend. Defaults
+ * to win32 backend.
+ */
+#define XRE_MAIN_FLAG_USE_METRO 0x01
+
+/**
  * Begin an XUL application. Does not return until the user exits the
  * application.
  *
  * @param argc/argv Command-line parameters to pass to the application. On
  *                  Windows, these should be in UTF8. On unix-like platforms
  *                  these are in the "native" character set.
  *
  * @param aAppData  Information about the application to be run.
  *
+ * @param aFlags    Platform specific flags.
+ *
  * @return         A native result code suitable for returning from main().
  *
  * @note           If the binary is linked against the standalone XPCOM glue,
  *                 XPCOMGlueStartup() should be called before this method.
- *
  */
 XRE_API(int,
-        XRE_main, (int argc, char* argv[], const nsXREAppData* sAppData))
+        XRE_main, (int argc, char* argv[], const nsXREAppData* aAppData,
+                   PRUint32 aFlags))
 
 /**
  * Given a path relative to the current working directory (or an absolute
  * path), return an appropriate nsILocalFile object.
  *
  * @note Pass UTF8 strings on Windows... native charset on other platforms.
  */
 XRE_API(nsresult,
@@ -424,13 +434,30 @@ XRE_API(void,
 #define XRE_HAS_DLL_BLOCKLIST
 XRE_API(void,
         XRE_SetupDllBlocklist, ())
 #endif
 
 XRE_API(void,
         XRE_TelemetryAccumulate, (int aID, PRUint32 aSample))
 
-
 XRE_API(void,
         XRE_InitOmnijar, (nsILocalFile* greOmni,
                           nsILocalFile* appOmni))
+
+#ifdef XP_WIN
+/**
+ * Valid environment types for XRE_GetWindowsEnvironment.
+ */
+enum WindowsEnvironmentType {
+  WindowsEnvironmentType_Desktop = 0,
+  WindowsEnvironmentType_Metro = 1
+};
+
+/**
+ * Retrieve the Windows desktop environment libXUL is running
+ * under. Valid after a call to XRE_main.
+ */
+XRE_API(WindowsEnvironmentType,
+        XRE_GetWindowsEnvironment, ())
+#endif // XP_WIN
+
 #endif // _nsXULAppAPI_h__

--- a/xulrunner/app/nsXULRunnerApp.cpp
+++ b/xulrunner/app/nsXULRunnerApp.cpp
@@ -342,10 +342,10 @@ int main(int argc, char* argv[])
   }
 
   AutoAppData appData(appDataLF);
   if (!appData) {
     Output(true, "Error: couldn't parse application.ini.\n");
     return 2;
   }
 
-  return XRE_main(argc, argv, appData);
+  return XRE_main(argc, argv, appData, 0);
 }

--- a/xulrunner/stub/nsXULStub.cpp
+++ b/xulrunner/stub/nsXULStub.cpp
@@ -475,17 +475,17 @@ main(int argc, char **argv)
       NS_NewLocalFile(NS_ConvertUTF8toUTF16(greDir), false,
                       &appData->xreDirectory);
 #else
       NS_NewNativeLocalFile(nsDependentCString(greDir), false,
                             &appData->xreDirectory);
 #endif
     }
 
-    retval = XRE_main(argc, argv, appData);
+    retval = XRE_main(argc, argv, appData, 0);
   }
 
   NS_LogTerm();
 
   XPCOMGlueShutdown();
 
   return retval;
 }