Maybe marginally so in isolation? But that isn't the metric that matters, since a browser has to render HTML. HTML + XUL is more complex than just HTML.

The article talks about XBL (the binding part of XUL?):

> There are hard to debug complications with binding lifecycles in our UI, and very few people know how it works.

> It adds enormous complexity to our platform in the frame constructor, style system, and the DOM implementation.