CustomEvent() currently does not serialize array data properly when sending event data across the XPCNativeWrapper to and from a web page and a Firefox extension.
Press a button to send a CustomEvent with the indicated data. In both cases the add-on will send back an event. See the two consoles for results.
WARNING: The normal button works a little, the first time. After that, both message-data styles get corrupted (detail is null, coming from the extension).