Poorly documented, implementation-defined, fragile, in-house only, etc. Mozilla can't hire anyone with deep knowledge of XUL to maintain XUL, and they can't as

Poorly documented, implementation-defined, fragile, in-house only, etc. Mozilla can't hire anyone with deep knowledge of XUL to maintain XUL, and they can't assume the current generation of experts will be around forever. They're trying to get rid of it. Writing more of it as anything but a stopgap would be counter to their current goals.