XHR progress event instances have an extra entry on their prototype chain (XMLHttpRequestProgressEvent) to provide some Gecko-compatibility properties ("totalSize" and "position", from LSProgressEvent originally).
Support for these progress event properties (and LSProgressEvent) has been dropped by Mozilla,
https://bugzilla.mozilla.org/show_bug.cgi?id=845631
Follow suit and deprecate&remove XMLHttpRequestProgressEvent. Perhaps outright removal, but a sensible first step would be to gauge usage.