diff --git a/gfx/thebes/gfxPlatform.cpp b/gfx/thebes/gfxPlatform.cpp
index 112a521f7a8..9a92ae54a10 100644
--- a/gfx/thebes/gfxPlatform.cpp
+++ b/gfx/thebes/gfxPlatform.cpp
@@ -2208,7 +2208,10 @@ gfxPlatform::NotifyCompositorCreated(LayersBackend aBackend)
     return;
   }
 
-  NS_ASSERTION(mCompositorBackend == LayersBackend::LAYERS_NONE, "Compositor backend changed.");
+  if (mCompositorBackend != LayersBackend::LAYERS_NONE) {
+    gfxCriticalNote << "Compositors might be mixed ("
+      << int(mCompositorBackend) << "," << int(aBackend) << ")";
+  }
 
   // Set the backend before we notify so it's available immediately.
   mCompositorBackend = aBackend;
diff --git a/widget/nsBaseWidget.cpp b/widget/nsBaseWidget.cpp
index 5b8965c17ee..59a368078d5 100644
--- a/widget/nsBaseWidget.cpp
+++ b/widget/nsBaseWidget.cpp
@@ -1132,7 +1132,11 @@ void nsBaseWidget::CreateCompositor(int aWidth, int aHeight)
 
   mLayerManager = lm.forget();
 
-  gfxPlatform::GetPlatform()->NotifyCompositorCreated(mLayerManager->GetCompositorBackendType());
+  if (mWindowType == eWindowType_toplevel) {
+    // Only track compositors for top-level windows, since other window types
+    // may use the basic compositor.
+    gfxPlatform::GetPlatform()->NotifyCompositorCreated(mLayerManager->GetCompositorBackendType());
+  }
 }
 
 bool nsBaseWidget::ShouldUseOffMainThreadCompositing()