Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check http://xhr.spec.whatwg.org/. Don't worry about it, it will not break TGE or any JIRA functionality.
Actually, synchronous AJAX calls cause this warning. They became deprecated due to possibility to block user interface in case of hanging of the server during the request processing.
JIRA can predict such types of errors and handle them respectively. That is why you can ignore this warning.