--- a/gfx/thebes/gfxPrefs.h
+++ b/gfx/thebes/gfxPrefs.h
@@ -379,17 +379,17 @@ private:
   DECL_GFX_PREF(Live, "gfx.color_management.rendering_intent", CMSRenderingIntent, int32_t, 0);
 
   DECL_GFX_PREF(Once, "gfx.device-reset.limit",                DeviceResetLimitCount, int32_t, 10);
   DECL_GFX_PREF(Once, "gfx.device-reset.threshold-ms",         DeviceResetThresholdMilliseconds, int32_t, -1);
 
   DECL_GFX_PREF(Once, "gfx.direct2d.disabled",                 Direct2DDisabled, bool, false);
   DECL_GFX_PREF(Once, "gfx.direct2d.force-enabled",            Direct2DForceEnabled, bool, false);
   DECL_GFX_PREF(Live, "gfx.direct3d11.reuse-decoder-device",   Direct3D11ReuseDecoderDevice, int32_t, -1);
-  DECL_GFX_PREF(Live, "gfx.direct3d11.allow-intel-mutex",      Direct3D11AllowIntelMutex, bool, true);
+  DECL_GFX_PREF(Live, "gfx.direct3d11.allow-intel-mutex",      Direct3D11AllowIntelMutex, bool, false);
   DECL_GFX_PREF(Live, "gfx.downloadable_fonts.keep_variation_tables", KeepVariationTables, bool, false);
   DECL_GFX_PREF(Live, "gfx.downloadable_fonts.otl_validation", ValidateOTLTables, bool, true);
   DECL_GFX_PREF(Live, "gfx.draw-color-bars",                   CompositorDrawColorBars, bool, false);
   DECL_GFX_PREF(Once, "gfx.e10s.hide-plugins-for-scroll",      HidePluginsForScroll, bool, true);
   DECL_GFX_PREF(Live, "gfx.layerscope.enabled",                LayerScopeEnabled, bool, false);
   DECL_GFX_PREF(Live, "gfx.layerscope.port",                   LayerScopePort, int32_t, 23456);
   // Note that        "gfx.logging.level" is defined in Logging.h.
   DECL_GFX_PREF(Live, "gfx.logging.level",                     GfxLoggingLevel, int32_t, mozilla::gfx::LOG_DEFAULT);