--- a/gfx/thebes/gfxPrefs.h
+++ b/gfx/thebes/gfxPrefs.h
@@ -541,16 +541,17 @@ private:
   DECL_GFX_PREF(Once, "layers.enable-tiles",                   LayersTilesEnabled, bool, false);
   DECL_GFX_PREF(Live, "layers.flash-borders",                  FlashLayerBorders, bool, false);
   DECL_GFX_PREF(Once, "layers.force-shmem-tiles",              ForceShmemTiles, bool, false);
   DECL_GFX_PREF(Live, "layers.frame-counter",                  DrawFrameCounter, bool, false);
   DECL_GFX_PREF(Once, "layers.gpu-process.enabled",            GPUProcessEnabled, bool, false);
   DECL_GFX_PREF(Once, "layers.gpu-process.force-enabled",      GPUProcessForceEnabled, bool, false);
   DECL_GFX_PREF(Once, "layers.gpu-process.timeout_ms",         GPUProcessTimeoutMs, int32_t, 5000);
   DECL_GFX_PREF(Live, "layers.gpu-process.max_restarts",       GPUProcessMaxRestarts, int32_t, 0);
+  DECL_GFX_PREF(Once, "layers.gpu-process.allow-software",     GPUProcessAllowSoftware, bool, false);
   DECL_GFX_PREF(Live, "layers.low-precision-buffer",           UseLowPrecisionBuffer, bool, false);
   DECL_GFX_PREF(Live, "layers.low-precision-opacity",          LowPrecisionOpacity, float, 1.0f);
   DECL_GFX_PREF(Live, "layers.low-precision-resolution",       LowPrecisionResolution, float, 0.25f);
   DECL_GFX_PREF(Live, "layers.max-active",                     MaxActiveLayers, int32_t, -1);
   DECL_GFX_PREF(Once, "layers.offmainthreadcomposition.force-disabled", LayersOffMainThreadCompositionForceDisabled, bool, false);
   DECL_GFX_PREF(Live, "layers.offmainthreadcomposition.frame-rate", LayersCompositionFrameRate, int32_t,-1);
   DECL_GFX_PREF(Live, "layers.orientation.sync.timeout",       OrientationSyncMillis, uint32_t, (uint32_t)0);
   DECL_GFX_PREF(Once, "layers.prefer-opengl",                  LayersPreferOpenGL, bool, false);

--- a/gfx/thebes/gfxWindowsPlatform.cpp
+++ b/gfx/thebes/gfxWindowsPlatform.cpp
@@ -1570,17 +1570,21 @@ gfxWindowsPlatform::InitGPUProcessSuppor
 {
   FeatureState& gpuProc = gfxConfig::GetFeature(Feature::GPU_PROCESS);
 
   if (!gpuProc.IsEnabled()) {
     return false;
   }
 
   if (!gfxConfig::IsEnabled(Feature::D3D11_COMPOSITING)) {
-    // Don't use the GPU process if not using D3D11.
+    // Don't use the GPU process if not using D3D11, unless software
+    // compositor is allowed
+    if (gfxPrefs::GPUProcessAllowSoftware()) {
+      return gpuProc.IsEnabled();
+    }
     gpuProc.Disable(
       FeatureStatus::Unavailable,
       "Not using GPU Process since D3D11 is unavailable",
       NS_LITERAL_CSTRING("FEATURE_FAILURE_NO_D3D11"));
   } else if (!IsWin7SP1OrLater()) {
     // On Windows 7 Pre-SP1, DXGI 1.2 is not available and remote presentation
     // for D3D11 will not work. Rather than take a regression we revert back
     // to in-process rendering.