diff --git a/dom/plugins/PluginInstanceChild.cpp b/dom/plugins/PluginInstanceChild.cpp
index 0c68d10ee61..3ef4f89debf 100644
--- a/dom/plugins/PluginInstanceChild.cpp
+++ b/dom/plugins/PluginInstanceChild.cpp
@@ -42,6 +42,8 @@
 #include "PluginStreamChild.h"
 #include "StreamNotifyChild.h"
 
+#include "mozilla/ipc/SyncChannel.h"
+
 using namespace mozilla::plugins;
 
 #ifdef MOZ_WIDGET_GTK2
@@ -578,6 +580,9 @@ PluginInstanceChild::PluginWindowProc(HWND hWnd,
                                       WPARAM wParam,
                                       LPARAM lParam)
 {
+    NS_ASSERTION(!mozilla::ipc::SyncChannel::IsPumpingMessages(),
+                 "Failed to prevent a nonqueued message from running!");
+
     PluginInstanceChild* self = reinterpret_cast<PluginInstanceChild*>(
         GetProp(hWnd, kPluginInstanceChildProperty));
     if (!self) {
diff --git a/ipc/glue/SyncChannel.cpp b/ipc/glue/SyncChannel.cpp
index 1b590e78914..7658d906bae 100644
--- a/ipc/glue/SyncChannel.cpp
+++ b/ipc/glue/SyncChannel.cpp
@@ -42,6 +42,11 @@
 
 #include "nsDebug.h"
 
+#ifdef OS_WIN
+#include "nsServiceManagerUtils.h"
+#include "nsIXULAppInfo.h"
+#endif
+
 using mozilla::MutexAutoLock;
 
 template<>
@@ -176,7 +181,368 @@ SyncChannel::OnChannelError()
 // Synchronization between worker and IO threads
 //
 
+namespace {
+bool gPumpingMessages = false;
+} // anonymous namespace
+
+// static
+bool
+SyncChannel::IsPumpingMessages()
+{
+    return gPumpingMessages;
+}
+
+#ifdef OS_WIN
+
+/**
+ * The Windows-only code below exists to solve a general problem with deadlocks
+ * that we experience when sending synchronous IPC messages to processes that
+ * contain native windows (i.e. HWNDs). Windows (the OS) sends synchronous
+ * messages between parent and child HWNDs in multiple circumstances (e.g.
+ * WM_PARENTNOTIFY, WM_NCACTIVATE, etc.), even when those HWNDs are controlled
+ * by different threads or different processes. Thus we can very easily end up
+ * in a deadlock by a call stack like the following:
+ *
+ * Process A:
+ *   - CreateWindow(...) creates a "parent" HWND.
+ *   - SendCreateChildWidget(HWND) is a sync IPC message that sends the "parent"
+ *         HWND over to Process B. Process A blocks until a response is received
+ *         from Process B.
+ *
+ * Process B:
+ *   - RecvCreateWidget(HWND) gets the "parent" HWND from Process A.
+ *   - CreateWindow(..., HWND) creates a "child" HWND with the parent from
+ *         process A.
+ *   - Windows (the OS) generates a WM_PARENTNOTIFY message that is sent
+ *         synchronously to Process A. Process B blocks until a response is
+ *         received from Process A. Process A, however, is blocked and cannot
+ *         process the message. Both processes are deadlocked.
+ *
+ * The example above has a few different workarounds (e.g. setting the
+ * WS_EX_NOPARENTNOTIFY style on the child window) but the general problem is
+ * persists. Once two HWNDs are parented we must not block their owning
+ * threads when manipulating either HWND.
+ *
+ * Windows requires any application that hosts native HWNDs to always process
+ * messages or risk deadlock. Given our architecture the only way to meet
+ * Windows' requirement and allow for synchronous IPC messages is to pump a
+ * miniature message loop during a sync IPC call. We avoid processing any
+ * queued messages during the loop, but "nonqueued" messages (see
+ * http://msdn.microsoft.com/en-us/library/ms644927(VS.85).aspx under the
+ * section "Nonqueued messages") cannot be avoided. Those messages are trapped
+ * in a special window procedure where we can either ignore the message or
+ * process it in some fashion.
+ */
+
+namespace {
+
+UINT gEventLoopMessage =
+    RegisterWindowMessage(L"SyncChannel::RunWindowsEventLoop Message");
+
+const wchar_t kOldWndProcProp[] = L"MozillaIPCOldWndProc";
+
+PRUnichar gAppMessageWindowName[256] = { 0 };
+PRInt32 gAppMessageWindowNameLength = 0;
+
+nsTArray<HWND>* gNeuteredWindows = nsnull;
+
+LRESULT CALLBACK
+NeuteredWindowProc(HWND hwnd,
+                   UINT uMsg,
+                   WPARAM wParam,
+                   LPARAM lParam)
+{
+    WNDPROC oldWndProc = (WNDPROC)GetProp(hwnd, kOldWndProcProp);
+    if (!oldWndProc) {
+        // We should really never ever get here.
+        NS_ERROR("No old wndproc!");
+        return DefWindowProc(hwnd, uMsg, wParam, lParam);
+    }
+
+    // XXX We may need more elaborate ways of faking some kinds of responses
+    //     here. For now we will let DefWindowProc handle the message, but in
+    //     the future we could allow the message to go to the intended wndproc
+    //     (via CallWindowProc) or cheat (via some combination of ReplyMessage
+    //     and PostMessage).
+
+#ifdef DEBUG
+    {
+        printf("WARNING: Received nonqueued message 0x%x during a sync IPC "
+               "message for window 0x%d", uMsg, hwnd);
+
+        wchar_t className[256] = { 0 };
+        if (GetClassNameW(hwnd, className, sizeof(className) - 1) > 0) {
+            printf(" (\"%S\")", className);
+        }
+
+        printf(", sending it to DefWindowProc instead of the normal "
+               "window procedure.\n");
+    }
+#endif
+    return DefWindowProc(hwnd, uMsg, wParam, lParam);
+}
+
+static inline bool
+WindowIsMozillaWindow(HWND hWnd)
+{
+    if (!IsWindow(hWnd)) {
+        NS_WARNING("Window has died!");
+        return false;
+    }
+
+    PRUnichar buffer[256] = { 0 };
+    int length = GetClassNameW(hWnd, (wchar_t*)buffer, sizeof(buffer) - 1);
+    if (length <= 0) {
+        NS_WARNING("Failed to get class name!");
+        return false;
+    }
+
+    nsDependentString className(buffer, length);
+    if (StringBeginsWith(className, NS_LITERAL_STRING("Mozilla")) ||
+        StringBeginsWith(className, NS_LITERAL_STRING("Gecko")) ||
+        className.EqualsLiteral("nsToolkitClass") ||
+        className.EqualsLiteral("nsAppShell:EventWindowClass")) {
+        return true;
+    }
+
+    // nsNativeAppSupport makes a window like "FirefoxMessageWindow" based on
+    // the toolkit app's name. It's pretty expensive to calculate this so we
+    // only try once.
+    if (gAppMessageWindowNameLength == 0) {
+        // This will only happen once.
+        nsCOMPtr<nsIXULAppInfo> appInfo =
+            do_GetService("@mozilla.org/xre/app-info;1");
+        if (appInfo) {
+            nsCAutoString appName;
+            if (NS_SUCCEEDED(appInfo->GetName(appName))) {
+                appName.Append("MessageWindow");
+                nsDependentString windowName(gAppMessageWindowName);
+                CopyUTF8toUTF16(appName, windowName);
+                gAppMessageWindowNameLength = windowName.Length();
+            }
+        }
+
+        // Don't try again if that failed.
+        if (gAppMessageWindowNameLength == 0) {
+            gAppMessageWindowNameLength = -1;
+        }
+    }
+
+    if (gAppMessageWindowNameLength != -1 &&
+        className.Equals(nsDependentString(gAppMessageWindowName,
+                                           gAppMessageWindowNameLength))) {
+        return true;
+    }
+
+    return false;
+}
+
+bool
+NeuterWindowProcedure(HWND hWnd)
+{
+    if (!WindowIsMozillaWindow(hWnd)) {
+        // Some other kind of window, skip.
+        return false;
+    }
+
+    NS_ASSERTION(!GetProp(hWnd, kOldWndProcProp),
+                 "This should always be null!");
+
+    // It's possible to get NULL out of SetWindowLongPtr, and the only way to
+    // know if that's a valid old value is to use GetLastError. Clear the error
+    // here so we can tell.
+    SetLastError(ERROR_SUCCESS);
+
+    LONG_PTR currentWndProc =
+        SetWindowLongPtr(hWnd, GWLP_WNDPROC, (LONG_PTR)NeuteredWindowProc);
+    if (!currentWndProc) {
+        if (ERROR_SUCCESS == GetLastError()) {
+            // No error, so we set something and must therefore reset it.
+            SetWindowLongPtr(hWnd, GWLP_WNDPROC, currentWndProc);
+        }
+        return false;
+    }
+
+    NS_ASSERTION(currentWndProc != (LONG_PTR)NeuteredWindowProc,
+                 "This shouldn't be possible!");
+
+    if (!SetProp(hWnd, kOldWndProcProp, (HANDLE)currentWndProc)) {
+        // Cleanup
+        NS_WARNING("SetProp failed!");
+        SetWindowLongPtr(hWnd, GWLP_WNDPROC, currentWndProc);
+        RemoveProp(hWnd, kOldWndProcProp);
+        return false;
+    }
+
+    return true;
+}
+
 void
+RestoreWindowProcedure(HWND hWnd)
+{
+    NS_ASSERTION(WindowIsMozillaWindow(hWnd), "Not a mozilla window!");
+
+    LONG_PTR oldWndProc = (LONG_PTR)RemoveProp(hWnd, kOldWndProcProp);
+    if (oldWndProc) {
+        NS_ASSERTION(oldWndProc != (LONG_PTR)NeuteredWindowProc,
+                     "This shouldn't be possible!");
+
+        LONG_PTR currentWndProc =
+            SetWindowLongPtr(hWnd, GWLP_WNDPROC, oldWndProc);
+        NS_ASSERTION(currentWndProc == (LONG_PTR)NeuteredWindowProc,
+                     "This should never be switched out from under us!");
+    }
+}
+
+LRESULT CALLBACK
+CallWindowProcedureHook(int nCode,
+                        WPARAM wParam,
+                        LPARAM lParam)
+{
+    if (nCode >= 0) {
+        NS_ASSERTION(gNeuteredWindows, "This should never be null!");
+
+        HWND hWnd = reinterpret_cast<CWPSTRUCT*>(lParam)->hwnd;
+
+        if (!gNeuteredWindows->Contains(hWnd) && NeuterWindowProcedure(hWnd)) {
+            if (!gNeuteredWindows->AppendElement(hWnd)) {
+                NS_ERROR("Out of memory!");
+                RestoreWindowProcedure(hWnd);
+            }
+        }
+    }
+    return CallNextHookEx(NULL, nCode, wParam, lParam);
+}
+
+} // anonymous namespace
+
+void
+SyncChannel::RunWindowsEventLoop()
+{
+    mMutex.AssertCurrentThreadOwns();
+
+    NS_ASSERTION(mEventLoopDepth >= 0, "Event loop depth mismatch!");
+
+    HHOOK windowHook = NULL;
+
+    nsAutoTArray<HWND, 20> neuteredWindows;
+
+    if (++mEventLoopDepth == 1) {
+        NS_ASSERTION(!gPumpingMessages, "Shouldn't be pumping already!");
+        gPumpingMessages = true;
+
+        if (!mUIThreadId) {
+            mUIThreadId = GetCurrentThreadId();
+        }
+        NS_ASSERTION(mUIThreadId, "ThreadId should not be 0!");
+
+        NS_ASSERTION(!gNeuteredWindows, "Should only set this once!");
+        gNeuteredWindows = &neuteredWindows;
+
+        windowHook = SetWindowsHookEx(WH_CALLWNDPROC, CallWindowProcedureHook,
+                                      NULL, mUIThreadId);
+        NS_ASSERTION(windowHook, "Failed to set hook!");
+    }
+
+    {
+        MutexAutoUnlock unlock(mMutex);
+
+        while (1) {
+            // Wait until we have a message in the queue. MSDN docs are a bit
+            // unclear, but it seems that windows from two different threads
+            // (and it should be noted that a thread in another process counts
+            // as a "different thread") will implicitly have their message
+            // queues attached if they are parented to one another. This wait
+            // call, then, will return for a message delivered to *either*
+            // thread.
+            DWORD result = MsgWaitForMultipleObjects(0, NULL, FALSE, INFINITE,
+                                                     QS_ALLINPUT);
+            if (result != WAIT_OBJECT_0) {
+                NS_ERROR("Wait failed!");
+                break;
+            }
+
+            // The only way to know which thread the message was
+            // delivered on is to use some logic on the return values of
+            // GetQueueStatus and PeekMessage. PeekMessage will return false
+            // if there are no "queued" messages, but it will run all
+            // "nonqueued" messages before returning. So if PeekMessage
+            // returns false and there are no "nonqueued" messages that were
+            // run then we know that the message we woke for was intended for
+            // a window on another thread.
+            bool haveSentMessagesPending = 
+                HIWORD(GetQueueStatus(QS_SENDMESSAGE)) & QS_SENDMESSAGE;
+
+            // This PeekMessage call will actually process all "nonqueued"
+            // messages that are pending before returning. If we have
+            // "nonqueued" messages pending then we should have switched out
+            // all the window procedures above. In that case this PeekMessage
+            // call won't actually cause any mozilla code (or plugin code) to
+            // run.
+
+            // We check first to see if we should break out of the loop by
+            // looking for the special message from the IO thread. We pull it
+            // out of the queue too.
+            MSG msg = { 0 };
+            if (PeekMessageW(&msg, (HWND)-1, gEventLoopMessage,
+                             gEventLoopMessage, PM_REMOVE)) {
+                break;
+            }
+
+            // If the following PeekMessage call fails to return a message for
+            // us (and returns false) and we didn't run any "nonqueued"
+            // messages then we must have woken up for a message designated for
+            // a window in another thread. If we loop immediately then we could
+            // enter a tight loop, so we'll give up our time slice here to let
+            // the child process its message.
+            if (!PeekMessageW(&msg, NULL, 0, 0, PM_NOREMOVE) &&
+                !haveSentMessagesPending) {
+                // Message was for child, we should wait a bit.
+                SwitchToThread();
+            }
+        }
+    }
+
+    NS_ASSERTION(mEventLoopDepth > 0, "Event loop depth mismatch!");
+
+    if (--mEventLoopDepth == 0) {
+        if (windowHook) {
+            UnhookWindowsHookEx(windowHook);
+        }
+
+        NS_ASSERTION(gNeuteredWindows == &neuteredWindows, "Bad pointer!");
+        gNeuteredWindows = nsnull;
+
+        PRUint32 count = neuteredWindows.Length();
+        for (PRUint32 index = 0; index < count; index++) {
+            RestoreWindowProcedure(neuteredWindows[index]);
+        }
+
+        gPumpingMessages = false;
+    }
+}
+
+inline void
+SyncChannel::WaitForNotify()
+{
+    RunWindowsEventLoop();
+}
+
+void
+SyncChannel::NotifyWorkerThread()
+{
+    mMutex.AssertCurrentThreadOwns();
+    NS_ASSERTION(mUIThreadId, "This should have been set already!");
+    if (!PostThreadMessage(mUIThreadId, gEventLoopMessage, 0, 0)) {
+        NS_WARNING("Failed to post thread message!");
+    }
+}
+
+//--------------------------------------------------
+// Not windows
+#else
+
+inline void
 SyncChannel::WaitForNotify()
 {
     mCvar.Wait();
@@ -188,6 +554,8 @@ SyncChannel::NotifyWorkerThread()
     mCvar.Notify();
 }
 
+#endif  // ifdef OS_WIN
+
 
 } // namespace ipc
 } // namespace mozilla
diff --git a/ipc/glue/SyncChannel.h b/ipc/glue/SyncChannel.h
index 245c65f5d56..db8e345dae6 100644
--- a/ipc/glue/SyncChannel.h
+++ b/ipc/glue/SyncChannel.h
@@ -40,8 +40,6 @@
 #ifndef ipc_glue_SyncChannel_h
 #define ipc_glue_SyncChannel_h 1
 
-#include <queue>
-
 #include "mozilla/ipc/AsyncChannel.h"
 
 namespace mozilla {
@@ -68,6 +66,10 @@ public:
         AsyncChannel(aListener),
         mPendingReply(0),
         mProcessingSyncMessage(false)
+#ifdef OS_WIN
+      , mUIThreadId(0)
+      , mEventLoopDepth(0)
+#endif
     {
     }
 
@@ -87,6 +89,8 @@ public:
     NS_OVERRIDE virtual void OnMessageReceived(const Message& msg);
     NS_OVERRIDE virtual void OnChannelError();
 
+    static bool IsPumpingMessages();
+
 protected:
     // Executed on the worker thread
     bool ProcessingSyncMessage() {
@@ -96,6 +100,10 @@ protected:
     void OnDispatchMessage(const Message& aMsg);
     void WaitForNotify();
 
+#ifdef OS_WIN
+    void RunWindowsEventLoop();
+#endif
+
     // Executed on the IO thread.
     void OnSendReply(Message* msg);
     void NotifyWorkerThread();
@@ -109,6 +117,11 @@ protected:
     MessageId mPendingReply;
     bool mProcessingSyncMessage;
     Message mRecvd;
+
+#ifdef OS_WIN
+    DWORD mUIThreadId;
+    int mEventLoopDepth;
+#endif
 };
 
 
diff --git a/widget/src/windows/Makefile.in b/widget/src/windows/Makefile.in
index d6fc33a77c6..c300992d8e6 100644
--- a/widget/src/windows/Makefile.in
+++ b/widget/src/windows/Makefile.in
@@ -132,6 +132,11 @@ ifndef WINCE
 ENABLE_CXX_EXCEPTIONS = 1
 endif
 
+ifdef MOZ_IPC
+include $(topsrcdir)/config/config.mk
+include $(topsrcdir)/ipc/chromium/chromium-config.mk
+endif
+
 include $(topsrcdir)/config/rules.mk
 
 CXXFLAGS += $(MOZ_CAIRO_CFLAGS)
diff --git a/widget/src/windows/nsAccelerometerWin.cpp b/widget/src/windows/nsAccelerometerWin.cpp
index f524c0706e0..bdf720f562a 100644
--- a/widget/src/windows/nsAccelerometerWin.cpp
+++ b/widget/src/windows/nsAccelerometerWin.cpp
@@ -410,7 +410,7 @@ ThinkPadSensor::~ThinkPadSensor()
 PRBool
 ThinkPadSensor::Startup()
 {
-  mLibrary = LoadLibrary("sensor.dll");
+  mLibrary = LoadLibraryW(L"sensor.dll");
   if (!mLibrary)
     return PR_FALSE;
 
diff --git a/widget/src/windows/nsDeviceContextSpecWin.cpp b/widget/src/windows/nsDeviceContextSpecWin.cpp
index 12368fdac31..d4dae5b281f 100644
--- a/widget/src/windows/nsDeviceContextSpecWin.cpp
+++ b/widget/src/windows/nsDeviceContextSpecWin.cpp
@@ -1005,7 +1005,9 @@ GlobalPrinters::EnumerateNativePrinters()
   PR_PL(("EnumerateNativePrinters\n"));
 
   TCHAR szDefaultPrinterName[1024];    
-  DWORD status = GetProfileString("devices", 0, ",", szDefaultPrinterName, sizeof(szDefaultPrinterName)/sizeof(TCHAR));
+  DWORD status = GetProfileString(TEXT("devices"), 0, TEXT(","),
+                                  szDefaultPrinterName,
+                                  NS_ARRAY_LENGTH(szDefaultPrinterName));
   if (status > 0) {
     DWORD count = 0;
     LPTSTR sPtr   = (LPTSTR)szDefaultPrinterName;
@@ -1036,7 +1038,9 @@ GlobalPrinters::GetDefaultPrinterName(LPTSTR& aDefaultPrinterName)
 #ifndef WINCE
   aDefaultPrinterName = nsnull;
   TCHAR szDefaultPrinterName[1024];    
-  DWORD status = GetProfileString("windows", "device", 0, szDefaultPrinterName, sizeof(szDefaultPrinterName)/sizeof(TCHAR));
+  DWORD status = GetProfileString(TEXT("windows"), TEXT("device"), 0,
+                                  szDefaultPrinterName,
+                                  NS_ARRAY_LENGTH(szDefaultPrinterName));
   if (status > 0) {
     TCHAR comma = (TCHAR)',';
     LPTSTR sPtr = (LPTSTR)szDefaultPrinterName;
@@ -1047,7 +1051,7 @@ GlobalPrinters::GetDefaultPrinterName(LPTSTR& aDefaultPrinterName)
     }
     aDefaultPrinterName = _tcsdup(szDefaultPrinterName);
   } else {
-    aDefaultPrinterName = _tcsdup("");
+    aDefaultPrinterName = _tcsdup(TEXT(""));
   }
 
   PR_PL(("DEFAULT PRINTER [%s]\n", aDefaultPrinterName));
diff --git a/widget/src/windows/nsWindow.cpp b/widget/src/windows/nsWindow.cpp
index 3b928d8bed8..1fbd41b0126 100644
--- a/widget/src/windows/nsWindow.cpp
+++ b/widget/src/windows/nsWindow.cpp
@@ -104,6 +104,10 @@
  **************************************************************
  **************************************************************/
 
+#ifdef MOZ_IPC
+#include "mozilla/ipc/SyncChannel.h"
+#endif
+
 #include "nsWindow.h"
 
 #include <windows.h>
@@ -3512,6 +3516,11 @@ PRBool nsWindow::ConvertStatus(nsEventStatus aStatus)
 // The WndProc procedure for all nsWindows in this toolkit
 LRESULT CALLBACK nsWindow::WindowProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
 {
+#ifdef MOZ_IPC
+  NS_ASSERTION(!mozilla::ipc::SyncChannel::IsPumpingMessages(),
+               "Failed to prevent a nonqueued message from running!");
+#endif
+
   // create this here so that we store the last rolled up popup until after
   // the event has been processed.
   nsAutoRollup autoRollup;