--- a/gfx/thebes/gfxPrefs.h
+++ b/gfx/thebes/gfxPrefs.h
@@ -518,16 +518,17 @@ private:
   DECL_GFX_PREF(Once, "layers.tile-pool-clear-timeout",        LayersTilePoolClearTimeout, uint32_t, (uint32_t)5000);
   DECL_GFX_PREF(Once, "layers.tiles.adjust",                   LayersTilesAdjust, bool, true);
   DECL_GFX_PREF(Once, "layers.tiles.edge-padding",             TileEdgePaddingEnabled, bool, true);
   DECL_GFX_PREF(Live, "layers.tiles.fade-in.enabled",          LayerTileFadeInEnabled, bool, false);
   DECL_GFX_PREF(Live, "layers.tiles.fade-in.duration-ms",      LayerTileFadeInDuration, uint32_t, 250);
   DECL_GFX_PREF(Live, "layers.transaction.warning-ms",         LayerTransactionWarning, uint32_t, 200);
   DECL_GFX_PREF(Once, "layers.uniformity-info",                UniformityInfo, bool, false);
   DECL_GFX_PREF(Once, "layers.use-image-offscreen-surfaces",   UseImageOffscreenSurfaces, bool, true);
+  DECL_GFX_PREF(Live, "layers.draw-mask-debug",                DrawMaskLayer, bool, false);
 
   DECL_GFX_PREF(Live, "layout.css.scroll-behavior.damping-ratio", ScrollBehaviorDampingRatio, float, 1.0f);
   DECL_GFX_PREF(Live, "layout.css.scroll-behavior.enabled",    ScrollBehaviorEnabled, bool, true);
   DECL_GFX_PREF(Live, "layout.css.scroll-behavior.spring-constant", ScrollBehaviorSpringConstant, float, 250.0f);
   DECL_GFX_PREF(Live, "layout.css.scroll-snap.prediction-max-velocity", ScrollSnapPredictionMaxVelocity, int32_t, 2000);
   DECL_GFX_PREF(Live, "layout.css.scroll-snap.prediction-sensitivity", ScrollSnapPredictionSensitivity, float, 0.750f);
   DECL_GFX_PREF(Live, "layout.css.scroll-snap.proximity-threshold", ScrollSnapProximityThreshold, int32_t, 200);
   DECL_GFX_PREF(Live, "layout.css.touch_action.enabled",       TouchActionEnabled, bool, false);

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -4606,16 +4606,17 @@ pref("layers.tiles.edge-padding", true);
 pref("layers.offmainthreadcomposition.async-animations", true);
 
 // Whether to log information about off main thread animations to stderr
 pref("layers.offmainthreadcomposition.log-animations", false);
 
 pref("layers.bufferrotation.enabled", true);
 
 pref("layers.componentalpha.enabled", true);
+pref("layers.draw-mask-debug", false);
 
 // Use the DT-backend implemented PushLayer
 pref("gfx.content.use-native-pushlayer", false);
 
 pref("gfx.content.always-paint", false);
 
 #ifdef ANDROID
 pref("gfx.apitrace.enabled",false);