--- a/dom/base/nsXMLHttpRequest.cpp
+++ b/dom/base/nsXMLHttpRequest.cpp
@@ -1985,17 +1985,22 @@ nsXMLHttpRequest::OnStartRequest(nsIRequ
       if (!(mState & XML_HTTP_REQUEST_ASYNC)) {
         // We don't make cool new features available in the bad synchronous
         // mode. The synchronous mode is for legacy only.
         mWarnAboutSyncHtml = true;
         mState &= ~XML_HTTP_REQUEST_PARSEBODY;
       } else {
         mIsHtml = true;
       }
-    } else if (type.Find("xml") == kNotFound) {
+    } else if (!(type.EqualsLiteral("text/xml") ||
+                 type.EqualsLiteral("application/xml") ||
+                 type.RFind("+xml", true, -1, 4) != kNotFound)) {
+      // Follow https://xhr.spec.whatwg.org/
+      // If final MIME type is not null, text/html, text/xml, application/xml,
+      // or does not end in +xml, return null.
       mState &= ~XML_HTTP_REQUEST_PARSEBODY;
     }
   } else {
     // The request failed, so we shouldn't be parsing anyway
     mState &= ~XML_HTTP_REQUEST_PARSEBODY;
   }
 
   if (mState & XML_HTTP_REQUEST_PARSEBODY) {