<p>How do you get around this Ajax cross site scripting problem on FireFox 3?</p>

If you're using jQuery it has a callback function to overcome this:
http://docs.jquery.com/Ajax/jQuery.ajax#options
As of jQuery 1.2, you can load JSON
  data located on another domain if you
  specify a JSONP callback, which can be
  done like so: "myurl?callback=?".
  jQuery automatically replaces the ?
  with the correct method name to call,
  calling your specified callback. Or,
  if you set the dataType to "jsonp" a
  callback will be automatically added
  to your Ajax request.
Alternatively you could make your ajax request to a server-side script which does the cross-domain call for you, then passes the data back to your script