XUL itself - of course.

Edit: you could argue that the solution to XUL should have been an upgrade to modern design rather than death, but that would have just been an expensive temporary reprieve, the world doesn’t stop changing,

I'm not sure what do you mean by that. No deep customization at all is, of course, easier to support than some.

I don't care about preserving the feel of XUL, or any aesthetics, but I do care about its role.

It's not about specific extensions and specific language. It's about the "before" allowing things like Conkeror and any kind of appearance change conceivable and the "after" - not, if we don't count stupid CSS that breaks with every update.