In order to use XUL in a recent FireFox, please install https://github.com/jvillalobos/Remote-XUL-Manager

from https://addons.mozilla.org/en-US/android/addon/remote-xul-manager/

and enable https://km-200.github.io/ (don't be bothered by the error message, OK.) to display XUL in FireFox ESR (or seamonkey, pale moon or waterfox or other REAL browsers instead of the limited nightly "FFox")

 NOW:

test right here : https://km-200.github.io/xul/index.xul - you will get an error message typically unless you have the add-on ! so there is no need to "inform us about it..." , understood?

this is a fork of : http://ted.mielczarek.org/code/mozilla/xuledit/xuledit.xul

all credits to : http://ted.mielczarek.org/code/mozilla