Make sure you set the signed.applets.codebase_principle_support parameter to true in about:config.
View the source of this page for details on how it was called. If you compile mozilla DEBUG, the message sent and received will be logged to the console.
This works by calling a SOAP service listed on X Methods Website using the low-level SOAP API in Mozilla.
Experimenters may wish to add other tests which exercize services, with specific user interfaces such as the one in this test.