Index: gc_core/js/helpers.js
==================================================================
--- gc_core/js/helpers.js
+++ gc_core/js/helpers.js
@@ -56,10 +56,11 @@
                 let { Cc, Ci } = require("chrome");
                 xRequest = Cc["@mozilla.org/xmlextras/xmlhttprequest;1"].createInstance();
                 xRequest.QueryInterface(Ci.nsIXMLHttpRequest);
             }
             xRequest.open('GET', spf, false); // 3rd arg is false for synchronous, sync is acceptable in workers
+            xRequest.overrideMimeType('text/json');
             xRequest.send();
             return xRequest.responseText;
         }
         catch (e) {
             this.logerror(e);