diff --git a/packages/api-utils/lib/content/content-proxy.js b/packages/api-utils/lib/content/content-proxy.js
index d1a37c5ec..db0dc9ec3 100644
--- a/packages/api-utils/lib/content/content-proxy.js
+++ b/packages/api-utils/lib/content/content-proxy.js
@@ -467,6 +467,17 @@ function handlerMaker(obj) {
         return getProxyForFunction(f, NativeFunctionWrapper(f));
       }
       
+      // Fix XPathResult's constants being undefined on XrayWrappers
+      // these constants are defined here:
+      // http://mxr.mozilla.org/mozilla-central/source/dom/interfaces/xpath/nsIDOMXPathResult.idl
+      // and are only numbers.
+      // See bug 665279 for platform fix progress
+      if (!o && typeof obj == "object" && name in Ci.nsIDOMXPathResult) {
+        let value = Ci.nsIDOMXPathResult[name];
+        if (typeof value == "number" && value === obj.wrappedJSObject[name])
+          return value;
+      }
+
       // Generic case
       return wrap(o, obj, name);
       
diff --git a/packages/api-utils/tests/test-content-proxy.js b/packages/api-utils/tests/test-content-proxy.js
index 68033abef..788ea00d2 100644
--- a/packages/api-utils/tests/test-content-proxy.js
+++ b/packages/api-utils/tests/test-content-proxy.js
@@ -196,6 +196,25 @@ exports.testProxy = function (test) {
         // that may break our proxy code
         test.assert(wrapped.XMLHttpRequest(), "we are able to instantiate XMLHttpRequest object");
         
+        // Check XPathResult bug with constants being undefined on 
+        // XPCNativeWrapper
+        let value = 
+          win.XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE;
+        let xpcXPathResult = XPCNativeWrapper(win).XPathResult;
+        test.assertEqual(xpcXPathResult.wrappedJSObject.
+                           UNORDERED_NODE_SNAPSHOT_TYPE, 
+                         value, 
+                         "XPathResult's constants are valid on unwrapped node");
+        // The following test will fail if platform is fixed,
+        // so we will be able to know when to remove the work around.
+        test.assertEqual(xpcXPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, 
+                         undefined, 
+                         "XPathResult's constants are undefined on " + 
+                         "XPCNativeWrapper (platform bug #)");
+        // Check that our work around is working:
+        test.assertEqual(wrapped.XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, 
+                         value, "XPathResult works correctly on Proxies");
+
         // Verify that inherited prototype function like initEvent 
         // are handled correctly. (e2.type will return an error if it's not the case)
         let event1 = document.createEvent( 'MouseEvents' );