--- a/gfx/layers/client/ContentClient.cpp
+++ b/gfx/layers/client/ContentClient.cpp
@@ -220,16 +220,20 @@ ContentClient::BeginPaint(PaintedLayer* 
     if (!newBuffer->Lock(lockMode)) {
       gfxCriticalNote << "Failed to lock new back buffer.";
       Clear();
       return result;
     }
 
     if (mBuffer) {
       newBuffer->UpdateDestinationFrom(*mBuffer, newBuffer->BufferRect());
+
+      // We are done with the old back buffer now and it is about to be
+      // destroyed, so unlock it.
+      mBuffer->Unlock();
     }
 
     // Ensure our reference to the front buffer is released
     // as well as the old back buffer.
     Clear();
 
     mBuffer = newBuffer;
   }