--- a/layout/base/FrameLayerBuilder.cpp
+++ b/layout/base/FrameLayerBuilder.cpp
@@ -4701,17 +4701,17 @@ FrameLayerBuilder::CheckInLayerTreeCompr
 }
 
 void
 ContainerState::CollectOldLayers()
 {
   for (Layer* layer = mContainerLayer->GetFirstChild(); layer;
        layer = layer->GetNextSibling()) {
     NS_ASSERTION(!layer->HasUserData(&gMaskLayerUserData),
-                 "Mask layer in layer tree; could not be recycled.");
+                 "Mask layers should not be part of the layer tree.");
     if (layer->HasUserData(&gPaintedDisplayItemLayerUserData)) {
       NS_ASSERTION(layer->AsPaintedLayer(), "Wrong layer type");
       mPaintedLayersAvailableForRecycling.PutEntry(static_cast<PaintedLayer*>(layer));
     }
 
     if (Layer* maskLayer = layer->GetMaskLayer()) {
       NS_ASSERTION(maskLayer->GetType() == Layer::TYPE_IMAGE,
                    "Could not recycle mask layer, unsupported layer type.");