--- a/dom/xhr/XMLHttpRequestWorker.cpp
+++ b/dom/xhr/XMLHttpRequestWorker.cpp
@@ -4,17 +4,16 @@
  * 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/. */
 
 #include "XMLHttpRequestWorker.h"
 
 #include "nsIDOMEvent.h"
 #include "nsIDOMEventListener.h"
 #include "nsIRunnable.h"
-#include "nsIXMLHttpRequest.h"
 #include "nsIXPConnect.h"
 
 #include "jsfriendapi.h"
 #include "js/TracingAPI.h"
 #include "js/GCPolicyAPI.h"
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/dom/Exceptions.h"
 #include "mozilla/dom/File.h"
@@ -296,17 +295,17 @@ const char* const sEventStrings[] = {
   // XMLHttpRequestEventTarget event types, supported by both XHR and Upload.
   "abort",
   "error",
   "load",
   "loadstart",
   "progress",
   "timeout",
 
-  // nsIXMLHttpRequest event types, supported only by XHR.
+  // XMLHttpRequest event types, supported only by XHR.
   "readystatechange",
   "loadend",
 };
 
 static_assert(MOZ_ARRAY_LENGTH(sEventStrings) == STRING_COUNT,
               "Bad string count!");
 
 class MainThreadProxyRunnable : public MainThreadWorkerSyncRunnable
@@ -1004,19 +1003,17 @@ Proxy::HandleEvent(nsIDOMEvent* aEvent)
     NS_WARNING("Failed to get target!");
     return NS_ERROR_FAILURE;
   }
 
   bool isUploadTarget = mXHR != target;
   ProgressEvent* progressEvent = aEvent->InternalDOMEvent()->AsProgressEvent();
 
   if (mInOpen && type.EqualsASCII(sEventStrings[STRING_readystatechange])) {
-    uint16_t readyState = 0;
-    if (NS_SUCCEEDED(mXHR->GetReadyState(&readyState)) &&
-        readyState == nsIXMLHttpRequest::OPENED) {
+    if (mXHR->ReadyState() == 1) {
       mInnerEventStreamId++;
     }
   }
 
   {
     AutoSafeJSContext cx;
     JSAutoRequest ar(cx);
 
@@ -1130,33 +1127,34 @@ EventRunnable::PreDispatch(WorkerPrivate
   JS::Rooted<JSObject*> scopeObj(cx, mScopeObj);
   // And reset mScopeObj now, before we have a chance to run its destructor on
   // some background thread.
   mScopeObj.reset();
 
   RefPtr<XMLHttpRequestMainThread>& xhr = mProxy->mXHR;
   MOZ_ASSERT(xhr);
 
-  if (NS_FAILED(xhr->GetResponseType(mResponseType))) {
-    MOZ_ASSERT(false, "This should never fail!");
-  }
+  const EnumEntry& entry =
+    XMLHttpRequestResponseTypeValues::strings[static_cast<uint32_t>(xhr->ResponseType())];
+  mResponseType.AssignASCII(entry.value, entry.length);
 
   ErrorResult rv;
   xhr->GetResponseText(mResponseText, rv);
   mResponseTextResult = rv.StealNSResult();
 
   if (NS_SUCCEEDED(mResponseTextResult)) {
     mResponseResult = mResponseTextResult;
     if (mResponseText.IsVoid()) {
       mResponse.setNull();
     }
   }
   else {
     JS::Rooted<JS::Value> response(cx);
-    mResponseResult = xhr->GetResponse(cx, &response);
+    xhr->GetResponse(cx, &response, rv);
+    mResponseResult = rv.StealNSResult();
     if (NS_SUCCEEDED(mResponseResult)) {
       if (!response.isGCThing()) {
         mResponse = response;
       } else {
         bool doClone = true;
         JS::Rooted<JS::Value> transferable(cx);
         JS::Rooted<JSObject*> obj(cx, response.isObject() ?
                                   &response.toObject() : nullptr);
@@ -1169,17 +1167,17 @@ EventRunnable::PreDispatch(WorkerPrivate
           } else {
             MOZ_ASSERT(!JS_IsDetachedArrayBufferObject(obj));
             JS::AutoValueArray<1> argv(cx);
             argv[0].set(response);
             obj = JS_NewArrayObject(cx, argv);
             if (obj) {
               transferable.setObject(*obj);
               // Only cache the response when the readyState is DONE.
-              if (xhr->ReadyState() == nsIXMLHttpRequest::DONE) {
+              if (xhr->ReadyState() == 4) {
                 mProxy->mArrayBufferResponseWasTransferred = true;
               }
             } else {
               mResponseResult = NS_ERROR_OUT_OF_MEMORY;
               doClone = false;
             }
           }
         }
@@ -1191,17 +1189,18 @@ EventRunnable::PreDispatch(WorkerPrivate
             mResponseResult = rv.StealNSResult();
             mProxy->mArrayBufferResponseWasTransferred = false;
           }
         }
       }
     }
   }
 
-  mStatusResult = xhr->GetStatus(&mStatus);
+  mStatus = xhr->GetStatus(rv);
+  mStatusResult = rv.StealNSResult();
 
   xhr->GetStatusText(mStatusText, rv);
   MOZ_ASSERT(!rv.Failed());
 
   mReadyState = xhr->ReadyState();
 
   xhr->GetResponseURL(mResponseURL);
 
@@ -1407,52 +1406,55 @@ AbortRunnable::RunOnMainThread(ErrorResu
 
 nsresult
 OpenRunnable::MainThreadRunInternal()
 {
   if (!mProxy->Init()) {
     return NS_ERROR_DOM_INVALID_STATE_ERR;
   }
 
-  nsresult rv;
-
   if (mBackgroundRequest) {
-    rv = mProxy->mXHR->SetMozBackgroundRequest(mBackgroundRequest);
+    nsresult rv = mProxy->mXHR->SetMozBackgroundRequest(mBackgroundRequest);
     NS_ENSURE_SUCCESS(rv, rv);
   }
 
+  ErrorResult rv;
+
   if (mWithCredentials) {
-    rv = mProxy->mXHR->SetWithCredentials(mWithCredentials);
-    NS_ENSURE_SUCCESS(rv, rv);
+    mProxy->mXHR->SetWithCredentials(mWithCredentials, rv);
+    if (NS_WARN_IF(rv.Failed())) {
+      return rv.StealNSResult();
+    }
   }
 
   if (mTimeout) {
-    rv = mProxy->mXHR->SetTimeout(mTimeout);
-    NS_ENSURE_SUCCESS(rv, rv);
+    mProxy->mXHR->SetTimeout(mTimeout, rv);
+    if (NS_WARN_IF(rv.Failed())) {
+      return rv.StealNSResult();
+    }
   }
 
   MOZ_ASSERT(!mProxy->mInOpen);
   mProxy->mInOpen = true;
 
-  ErrorResult rv2;
   mProxy->mXHR->Open(mMethod, mURL, true,
                      mUser.WasPassed() ? mUser.Value() : VoidString(),
                      mPassword.WasPassed() ? mPassword.Value() : VoidString(),
-                     rv2);
+                     rv);
 
   MOZ_ASSERT(mProxy->mInOpen);
   mProxy->mInOpen = false;
 
-  if (rv2.Failed()) {
-    return rv2.StealNSResult();
+  if (NS_WARN_IF(rv.Failed())) {
+    return rv.StealNSResult();
   }
 
-  mProxy->mXHR->SetResponseType(mResponseType, rv2);
-  if (rv2.Failed()) {
-    return rv2.StealNSResult();
+  mProxy->mXHR->SetResponseType(mResponseType, rv);
+  if (NS_WARN_IF(rv.Failed())) {
+    return rv.StealNSResult();
   }
 
   return NS_OK;
 }
 
 void
 SendRunnable::RunOnMainThread(ErrorResult& aRv)
 {
@@ -2181,20 +2183,20 @@ XMLHttpRequestWorker::Abort(ErrorResult&
 
   if (!mProxy) {
     return;
   }
 
   // Set our status to 0 and statusText to "" if we
   // will be aborting an ongoing fetch, so the upcoming
   // abort events we dispatch have the correct info.
-  if ((mStateData.mReadyState == nsIXMLHttpRequest::OPENED && mStateData.mFlagSend) ||
-      mStateData.mReadyState == nsIXMLHttpRequest::HEADERS_RECEIVED ||
-      mStateData.mReadyState == nsIXMLHttpRequest::LOADING ||
-      mStateData.mReadyState == nsIXMLHttpRequest::DONE) {
+  if ((mStateData.mReadyState == State::opened && mStateData.mFlagSend) ||
+      mStateData.mReadyState == State::headers_received ||
+      mStateData.mReadyState == State::loading ||
+      mStateData.mReadyState == State::done) {
     mStateData.mStatus = 0;
     mStateData.mStatusText.Truncate();
   }
 
   MaybeDispatchPrematureAbortEvents(aRv);
   if (aRv.Failed()) {
     return;
   }
@@ -2277,17 +2279,17 @@ XMLHttpRequestWorker::OverrideMimeType(c
 
   // We're supposed to throw if the state is not OPENED or HEADERS_RECEIVED. We
   // can detect OPENED really easily but we can't detect HEADERS_RECEIVED in a
   // non-racy way until the XHR state machine actually runs on this thread
   // (bug 671047). For now we're going to let this work only if the Send()
   // method has not been called, unless the send has been aborted.
   if (!mProxy || (SendInProgress() &&
                   (mProxy->mSeenLoadStart ||
-                   mStateData.mReadyState > nsIXMLHttpRequest::OPENED))) {
+                   mStateData.mReadyState > 1))) {
     aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
     return;
   }
 
   RefPtr<OverrideMimeTypeRunnable> runnable =
     new OverrideMimeTypeRunnable(mWorkerPrivate, mProxy, aMimeType);
   runnable->Dispatch(Terminating, aRv);
 }
@@ -2313,17 +2315,17 @@ XMLHttpRequestWorker::SetResponseType(XM
     // Open() has not been called yet. We store the responseType and we will use
     // it later in Open().
     mResponseType = aResponseType;
     return;
   }
 
   if (SendInProgress() &&
       (mProxy->mSeenLoadStart ||
-       mStateData.mReadyState > nsIXMLHttpRequest::OPENED)) {
+       mStateData.mReadyState > 1)) {
     aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
     return;
   }
 
   RefPtr<SetResponseTypeRunnable> runnable =
     new SetResponseTypeRunnable(mWorkerPrivate, mProxy, aResponseType);
   runnable->Dispatch(Terminating, aRv);
   if (aRv.Failed()) {