changes as per review comments

components/script/dom/mutationobserver.rs

@@ -4,11 +4,12 @@
 
 use core::ptr::null;
 use dom;
+use dom::bindings::codegen::Bindings::MutationObserverBinding;
 use dom::bindings::codegen::Bindings::MutationObserverBinding::MutationCallback;
 use dom::bindings::codegen::Bindings::MutationObserverBinding::MutationObserverInit;
 use dom::bindings::error::Fallible;
 use dom::bindings::js::Root;
-use dom::bindings::reflector::Reflector;
+use dom::bindings::reflector::{Reflector, reflect_dom_object};
 use dom::bindings::trace::JSTraceable;
 use dom::mutationrecord::MutationRecord;
 use dom::node::Node;
@@ -21,11 +22,17 @@ use std::rc::Rc;
 #[dom_struct]
 pub struct MutationObserver {
     reflector_: Reflector,
-    callback: MutationCallback,
+    #[ignore_heap_size_of = "can't measure Rc values"]
+    callback: Rc<MutationCallback>,
 }
 
 impl MutationObserver {
-    fn new(global: &Window, callback: Rc<MutationCallback>) -> MutationObserver {
+    pub fn new(global: &Window, callback: Rc<MutationCallback>) -> Root<MutationObserver> {
+        let boxed_observer = box MutationObserver::new_inherited(callback);
+        reflect_dom_object(boxed_observer, global, MutationObserverBinding::Wrap)
+    }
+
+    pub fn new_inherited(callback: Rc<MutationCallback>) -> MutationObserver {
         MutationObserver {
             reflector_: Reflector::new(),
             callback: callback,
@@ -34,7 +41,7 @@ impl MutationObserver {
 
     pub fn Constructor(global: &Window, callback: Rc<MutationCallback>) -> Fallible<Root<MutationObserver>> {
         let observer = MutationObserver::new(global, callback);
-        ScriptThread::add_mutation_observer(Root::from_ref(&*observer))
+        ScriptThread::add_mutation_observer(observer.deref());
+        return Ok(observer);
     }
-
 }

components/script/dom/mutationrecord.rs

@@ -37,7 +37,7 @@ impl MutationRecord {
 impl MutationRecordMethods for MutationRecord {
     // https://dom.spec.whatwg.org/#dom-mutationrecord-type
     fn Type(&self) -> DOMString {
-        return self.record_type;
+        DOMString::from(self.record_type.clone())
     }
 
     // https://dom.spec.whatwg.org/#dom-mutationrecord-target

components/script/dom/webidls/MutationObserver.webidl

@@ -1,3 +1,6 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 /*
  * The origin of this IDL file is:
  * https://dom.spec.whatwg.org/#mutationobserver

components/script/dom/webidls/MutationRecord.webidl

@@ -1,3 +1,6 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 /*
  * The origin of this IDL file is:
  * https://dom.spec.whatwg.org/#mutationrecord

components/script/script_thread.rs

@@ -18,6 +18,7 @@
 //! loop.
 
 use bluetooth_traits::BluetoothRequest;
+use core::borrow::BorrowMut;
 use devtools;
 use devtools_traits::{DevtoolScriptControlMsg, DevtoolsPageInfo};
 use devtools_traits::{ScriptToDevtoolsControlMsg, WorkerId};
@@ -575,10 +576,11 @@ impl ScriptThreadFactory for ScriptThread {
 
 impl ScriptThread {
     pub fn add_mutation_observer(observer: &MutationObserver) {
-        SCRIPT_THREAD_ROOT.with(|root| {
+       SCRIPT_THREAD_ROOT.with(|root| {
             let script_thread = unsafe { &*root.get().unwrap() };
             script_thread.mutation_observers
-                .push(JS::from_ref(&*observer));
+                .borrow_mut()
+                .push(JS::from_ref(observer));
         })
     }
 

resources/prefs.json

@@ -3,8 +3,8 @@
   "dom.bluetooth.testing.enabled": false,
   "dom.forcetouch.enabled": false,
   "dom.mouseevent.which.enabled": false,
-  "dom.mutation_observer.enabled": false,
   "dom.mozbrowser.enabled": false,
+  "dom.mutation_observer.enabled": false,
   "dom.permissions.enabled": false,
   "dom.permissions.testing.allowed_in_nonsecure_contexts": false,
   "dom.serviceworker.timeout_seconds": 60,