--- a/dom/plugins/ipc/PluginModuleParent.cpp
+++ b/dom/plugins/ipc/PluginModuleParent.cpp
@@ -666,16 +666,17 @@ PluginModuleParent::PluginModuleParent(b
     , mShutdown(false)
     , mHadLocalInstance(false)
     , mClearSiteDataSupported(false)
     , mGetSitesWithDataSupported(false)
     , mNPNIface(nullptr)
     , mNPPIface(nullptr)
     , mPlugin(nullptr)
     , mTaskFactory(this)
+    , mSandboxLevel(0)
     , mIsFlashPlugin(false)
     , mIsStartingAsync(false)
     , mNPInitialized(false)
     , mIsNPShutdownPending(false)
     , mAsyncNewRv(NS_ERROR_NOT_INITIALIZED)
 {
 #if defined(XP_WIN) || defined(XP_MACOSX) || defined(MOZ_WIDGET_GTK)
     mIsStartingAsync = aAllowAsyncInit &&
@@ -728,17 +729,16 @@ PluginModuleChromeParent::PluginModuleCh
     , mChromeTaskFactory(this)
     , mHangAnnotationFlags(0)
     , mHangAnnotatorMutex("PluginModuleChromeParent::mHangAnnotatorMutex")
 #ifdef XP_WIN
     , mPluginCpuUsageOnHang()
     , mHangUIParent(nullptr)
     , mHangUIEnabled(true)
     , mIsTimerReset(true)
-    , mSandboxLevel(aSandboxLevel)
 #ifdef MOZ_CRASHREPORTER
     , mCrashReporterMutex("PluginModuleChromeParent::mCrashReporterMutex")
     , mCrashReporter(nullptr)
 #endif
 #endif
 #ifdef MOZ_CRASHREPORTER_INJECTOR
     , mFlashProcess1(0)
     , mFlashProcess2(0)
@@ -746,16 +746,17 @@ PluginModuleChromeParent::PluginModuleCh
 #endif
     , mInitOnAsyncConnect(false)
     , mAsyncInitRv(NS_ERROR_NOT_INITIALIZED)
     , mAsyncInitError(NPERR_NO_ERROR)
     , mContentParent(nullptr)
 {
     NS_ASSERTION(mSubprocess, "Out of memory!");
     sInstantiated = true;
+    mSandboxLevel = aSandboxLevel;
     mRunID = GeckoChildProcessHost::GetUniqueID();
 
 #ifdef MOZ_ENABLE_PROFILER_SPS
     InitPluginProfiling();
 #endif
 
     mozilla::HangMonitor::RegisterAnnotator(*this);
 }
@@ -2661,16 +2662,33 @@ PluginModuleParent::NPP_NewInternal(NPMI
     }
 
     nsDependentCString strPluginType(pluginType);
     PluginInstanceParent* parentInstance =
         new PluginInstanceParent(this, instance, strPluginType, mNPNIface);
 
     if (mIsFlashPlugin) {
         parentInstance->InitMetadata(strPluginType, srcAttribute);
+#ifdef XP_WIN
+        // Force windowless mode (bug 1201904) when sandbox level >= 2
+        if (mSandboxLevel >= 2) {
+           NS_NAMED_LITERAL_CSTRING(wmodeAttributeName, "wmode");
+           NS_NAMED_LITERAL_CSTRING(opaqueAttributeValue, "opaque");
+           auto wmodeAttributeIndex =
+               names.IndexOf(wmodeAttributeName, 0, comparator);
+           if (wmodeAttributeIndex != names.NoIndex) {
+               if (!values[wmodeAttributeIndex].EqualsLiteral("transparent")) {
+                   values[wmodeAttributeIndex].Assign(opaqueAttributeValue);
+               }
+           } else {
+               names.AppendElement(wmodeAttributeName);
+               values.AppendElement(opaqueAttributeValue);
+           }
+        }
+#endif
     }
 
     // Release the surrogate reference that was in pdata
     RefPtr<PluginAsyncSurrogate> surrogate(
         dont_AddRef(PluginAsyncSurrogate::Cast(instance)));
     // Now replace it with the instance
     instance->pdata = static_cast<PluginDataResolver*>(parentInstance);
 

--- a/dom/plugins/ipc/PluginModuleParent.h
+++ b/dom/plugins/ipc/PluginModuleParent.h
@@ -320,16 +320,17 @@ protected:
     TaskFactory<PluginModuleParent> mTaskFactory;
     nsString mPluginDumpID;
     nsString mBrowserDumpID;
     nsString mHangID;
     RefPtr<nsIObserver> mProfilerObserver;
     TimeDuration mTimeBlocked;
     nsCString mPluginName;
     nsCString mPluginVersion;
+    int32_t mSandboxLevel;
     bool mIsFlashPlugin;
 
 #ifdef MOZ_X11
     // Dup of plugin's X socket, used to scope its resources to this
     // object instead of the plugin process's lifetime
     ScopedClose mPluginXSocketFdDup;
 #endif
 
@@ -543,17 +544,16 @@ private:
     Atomic<uint32_t> mHangAnnotationFlags;
     mozilla::Mutex mHangAnnotatorMutex;
     InfallibleTArray<mozilla::ipc::IProtocol*> mProtocolCallStack;
 #ifdef XP_WIN
     InfallibleTArray<float> mPluginCpuUsageOnHang;
     PluginHangUIParent *mHangUIParent;
     bool mHangUIEnabled;
     bool mIsTimerReset;
-    int32_t mSandboxLevel;
 #ifdef MOZ_CRASHREPORTER
     /**
      * This mutex protects the crash reporter when the Plugin Hang UI event
      * handler is executing off main thread. It is intended to protect both
      * the mCrashReporter variable in addition to the CrashReporterParent object
      * that mCrashReporter refers to.
      */
     mozilla::Mutex mCrashReporterMutex;