[jQuery] ajax request on local machine and FF3/Firebug "Access to restricted URI denied code: 1012"

[jquertil]

this has been discussed before, but I'm still hazy:

In FF2 and old Firebug this function ran fine with local files (no
local web server, just straight-up file on local desktop).

Now Firebug complains "Access to restricted URI denied code: 1012"

  $.ajax({
    type: "POST",
    url: myFileName,
    data: {},
    dataType: "jsonp",
    async: false,
    success: function(ret){
    JSON=eval('('+ret+')');
    }
});

I added the line
     dataType:"jsonp"
after reading up on this inside the list archives. This did nothing.

I don't feel like running all my stuff on a local web server just so I
can do some simple ajax - am I stuck? Should I downgrade to older
Firebug?

Any pointers much appreciated!