A polyfix for https://bugzil.la/697151
Just include FirefoxSyncXHRPolyfix.js in your web-page before any scripts which use XMLHttpRequests:
<script src="FirefoxSyncXHRPolyfix.js"></script>
This polyfix tries to mitigate a Firefox interop issue wherein async XHR events and postMessages can fire before an ongoing sync XHRs' events are handled (https://bugzil.la/697151).
This is designed to only actually polyfix when necessary, and to not use any features which would cause parsing errors on older browsers, and to use as few modern JS-isms as possible so that it is backward-compatible down to Firefox 6 (which has WeakMap).
In a nutshell, the fix blocks async XHR events and postMessages from firing while any sync XHR is ongoing, to match what other browsers do.
Note however that there is no interop on which order events will fire after the sync XHR completes. Any blocked async XHR events may fire after a setTimeout, rAf, or postMessage callback, for instance. As such the polyfix only assures that any blocked events call back in the order in which they were blocked.