--- a/layout/painting/FrameLayerBuilder.cpp
+++ b/layout/painting/FrameLayerBuilder.cpp
@@ -767,17 +767,17 @@ public:
    * All the display items that have been assigned to this painted layer.
    * These items get added by Accumulate().
    */
   nsTArray<AssignedDisplayItem> mAssignedDisplayItems;
   /**
    * Tracks the active opacity markers by holding the indices to PUSH_OPACITY
    * items in |mAssignedDisplayItems|.
    */
-  nsTArray<size_t> mOpacityIndices;
+  AutoTArray<size_t, 2> mOpacityIndices;
 };
 
 struct NewLayerEntry {
   NewLayerEntry()
     : mAnimatedGeometryRoot(nullptr)
     , mASR(nullptr)
     , mClipChain(nullptr)
     , mScrollMetadataASR(nullptr)