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It's never been a better time to switch to Firefox
(www.androidpolice.com)
This is a most excellent place for technology news and articles.
If I remember correctly, XUL/XBL is dead. They removed that code a while back after they transitioned to WebExtensions. The current frontend is HTML, CSS and JavaScript.
There are still bits of XUL around but I believe the preference is to use HTML elements wherever possible and they've been stripping XUL elements out.