Your PG fails on firefox due to the data media type. It must be: data:text/xml instead of data:text.
If I understand your question correctly, you want to create multiple GUI instances without having to reload the XML over HTTP every time. Each instance with a different context.
Here is an example of what I think would work for you:
In this case, the XML is only ever loaded once. It must be reparsed for every GUI instantiation, but I believe that is fundamental to the design of XmlLoader.