I just spent some time debugging the following. If Internet Explorer's "work offline" button is ticked, then when starting some JNLP under Firefox, I can't connect to anything via webstart? I was trying to run bugster, and I keep getting errors about the server being unavailable.

    Unable to launch the application.

details:

    System is offline and the application does not specify
    <offline-allowed/>
Duplicated on a second XP box and Windows 7. Hopefully that is enough to get you going. Let me know if not.