--- a/dom/media/platforms/PDMFactory.cpp
+++ b/dom/media/platforms/PDMFactory.cpp
@@ -41,16 +41,20 @@
 #include "AgnosticDecoderModule.h"
 #include "EMEDecoderModule.h"
 
 #include "DecoderDoctorDiagnostics.h"
 
 #include "MP4Decoder.h"
 #include "mozilla/dom/RemoteVideoDecoder.h"
 
+#ifdef XP_WIN
+#include "mozilla/WindowsVersion.h"
+#endif
+
 #include "mp4_demuxer/H264.h"
 
 namespace mozilla {
 
 extern already_AddRefed<PlatformDecoderModule> CreateAgnosticDecoderModule();
 extern already_AddRefed<PlatformDecoderModule> CreateBlankDecoderModule();
 
 class PDMFactoryImpl final {
@@ -313,17 +317,24 @@ PDMFactory::CreatePDMs()
 #ifdef MOZ_WIDGET_ANDROID
   if(MediaPrefs::PDMAndroidMediaCodecPreferred() &&
      MediaPrefs::PDMAndroidMediaCodecEnabled()) {
     m = new AndroidDecoderModule();
     StartupPDM(m);
   }
 #endif
 #ifdef XP_WIN
-  if (MediaPrefs::PDMWMFEnabled()) {
+  if (MediaPrefs::PDMWMFEnabled() && IsVistaOrLater()) {
+    // *Only* use WMF on Vista and later, as if Firefox is run in Windows 95
+    // compatibility mode on Windows 7 (it does happen!) we may crash trying
+    // to startup WMF. So we need to detect the OS version here, as in
+    // compatibility mode IsVistaOrLater() and friends behave as if we're on
+    // the emulated version of Windows. See bug 1279171.
+    // Additionally, we don't want to start the RemoteDecoderModule if we
+    // expect it's not going to work (i.e. on Windows older than Vista).
     m = new WMFDecoderModule();
     RefPtr<PlatformDecoderModule> remote = new dom::RemoteDecoderModule(m);
     mWMFFailedToLoad = !StartupPDM(remote);
     if (mWMFFailedToLoad) {
       mWMFFailedToLoad = !StartupPDM(m);
     }
   } else {
     mWMFFailedToLoad = MediaPrefs::DecoderDoctorWMFDisabledIsFailure();

--- a/dom/media/platforms/wmf/WMFUtils.cpp
+++ b/dom/media/platforms/wmf/WMFUtils.cpp
@@ -197,16 +197,25 @@ LoadDLLs()
   ENSURE_FUNCTION_PTR_HELPER(FunctionName##Ptr_t, FunctionName, DLL) \
 
 #define DECL_FUNCTION_PTR(FunctionName, ...) \
   typedef HRESULT (STDMETHODCALLTYPE * FunctionName##Ptr_t)(__VA_ARGS__)
 
 HRESULT
 MFStartup()
 {
+  if (!IsVistaOrLater()) {
+    // *Only* use WMF on Vista and later, as if Firefox is run in Windows 95
+    // compatibility mode on Windows 7 (it does happen!) we may crash trying
+    // to startup WMF. So we need to detect the OS version here, as in
+    // compatibility mode IsVistaOrLater() and friends behave as if we're on
+    // the emulated version of Windows. See bug 1279171.
+    return E_FAIL;
+  }
+
   HRESULT hr = LoadDLLs();
   NS_ENSURE_TRUE(SUCCEEDED(hr), hr);
 
   const int MF_VISTA_VERSION = (0x0001 << 16 | MF_API_VERSION);
   const int MF_WIN7_VERSION = (0x0002 << 16 | MF_API_VERSION);
 
   // decltype is unusable for functions having default parameters
   DECL_FUNCTION_PTR(MFStartup, ULONG, DWORD);