JQuery $.ajax(), $.post sending “OPTIONS” as REQUEST_METHOD in Firefox

Having trouble with what I thought was a relatively simple jQuery plugin…

The plugin should fetch data from a php script via ajax to add options to a <select>. The ajax request is pretty generic:

$.ajax({
  url: o.url,
  type: 'post',
  contentType: "application/x-www-form-urlencoded",
  data: '{"method":"getStates", "program":"EXPLORE"}',
  success: function (data, status) {
    console.log("Success!!");
    console.log(data);
    console.log(status);
  },
  error: function (xhr, desc, err) {
    console.log(xhr);
    console.log("Desc: " + desc + "\nErr:" + err);
  }
});

This seems to work fine in Safari. In Firefox 3.5, the REQUEST_TYPE on the server is always 'OPTIONS', and the $_POST data does not appear. Apache logs the request as type 'OPTIONS':

::1 - - [08/Jul/2009:11:43:27 -0500] "OPTIONS sitecodes.php HTTP/1.1" 200 46

Why would this ajax call work in Safari, but not Firefox, and how do I fix it for Firefox?

Response Headers
Date: Wed, 08 Jul 2009 21:22:17 GMT
Server:Apache/2.0.59 (Unix) PHP/5.2.6 DAV/2
X-Powered-By: PHP/5.2.6
Content-Length  46
Keep-Alive  timeout=15, max=100
Connection  Keep-Alive
Content-Type    text/html

Request Headers
Host    orderform:8888
User-Agent  Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1) Gecko/20090624 Firefox/3.5
Accept  text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language en-us,en;q=0.5
Accept-Encoding gzip,deflate
Accept-Charset  ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive  300
Connection  keep-alive
Origin  http://ux.inetu.act.org
Access-Control-Request-Method   POST
Access-Control-Request-Headers  x-requested-with

Here is a picture of the Firebug output: