Firefox says could not download the search plugin from

Backstory:

I'm trying to dynamically generate a search plugin for Firefox based on user entered values. I'm not including the forms and cruft surrounding it, because I've narrowed it down to a simple failing test case trying to import any XML.

Code:

Simplified JS

var browserSearchService = Components
        .classes["@http://ift.tt/1nSxH3Q"]
        .getService(Components.interfaces.nsIBrowserSearchService);

var EngineProperties = {
                xml :   'http://localhost/search.xml',
                dataType: 3,
                iconURL : 'http://localhost/logo.png',
                confirm : false,
                callback : function addEngineCallback(){
                    console.log('Jason is the greatest');           
                }
            }

browserSearchService.addEngine( EngineProperties.xml,
                                            EngineProperties.dataType,
                                            EngineProperties.iconURL,
                                            EngineProperties.confirm,
                                            EngineProperties.callback);

Actual XML

<?xml version="1.0" encoding="UTF-8"?>
<OpenSearchDescription xmlns="http://ift.tt/LMPdQR"
                       xmlns:moz="http://ift.tt/1jZxGJz">
  <ShortName>Jason</ShortName>
  <Description>Powered By Jason</Description>
  <InputEncoding>UTF-8</InputEncoding>
  <Image width="16" height="16" type="image/x-icon">http://localhost/logo.png</Image>

  <URL method="get" type="text/html" template="http://ift.tt/1nSxHkd}" />
  <URL method="get" type="application/x-moz-keywordsearch" 
    template="http://ift.tt/1ogQT6y}" />
  <Url method="get" type="application/x-suggestions+json" 
    template="http://ift.tt/1nSxGwG"/>

  <moz:SearchForm>http://ift.tt/1ogQQYk;
</OpenSearchDescription>

From what I've seen the error should indicate an invalid XML file, but for the life of me I can't find anything wrong with this. I've loaded it in Firefox fixed all the typos and syntactical errors I found (used to have & instead of &, and the browser displays and parses it fine, but yet it won't load as an open search search engine.

Does FF not support localhost, maybe? I'm drawing a blank here.

Thanks in advance for any insight!