Firefox seems to mis-handle $.ajax POST to REST-style url web service

Firefox seems to mis-handle $.ajax POST to REST-style url web service

I am successfully using $.ajax to POST data to a REST-style web service but Firefox 3.6 and Chrome 7 return to my error:fn() while IE 8 returns to my success:fn().  The jQuery in all three browsers puts the user-supplied data in the database where it should be.  Here is the $.ajax part of my code:

$.ajax({
          type:"POST",
          url:'http://localhost:52775/Service1.svc/myPost?x=' + $('input#boxInput').val(),
          contentType: "text/plain",
          success:function(xml) {   alert('returned = ' + xml.text);  },
          error:function(theData) { alert('returned = ' + theData.text); }
         } );

The web service has to have a POST. It gets its data from the querystring. That, ofcourse, is required for REST compliance. In addition to hitting the error:fn() even tho the data submit is successful; the alert that popsup reads, "returned = undefined"  I guess that problem goes away if the POSTs from Firefox and Chrome return to my success:fn().  Ideas?

SideNote:  Until I put the contentType: option in there the data was not getting in the db from Firefox.  I guess there was this access control issue for Firefox.