It's not about XUL, it's about having no limit on what you can do to the browser. Old-school extensions can change entire parts of the browser, remove features

It's not about XUL, it's about having no limit on what you can do to the browser. Old-school extensions can change entire parts of the browser, remove features, there really is no limit because the extension code is dumped inside the browser code without any kind of interface or security limitation. So it's powerful, but potentially destructive as a bug in an extension can impact the whole browser, and same goes for performances. On the other hand, the new API is more like Chrome where you have defined entry points to plug your extension. The new API approach has limitations, but it's a much better way to do extensions. Not only it limits the damage a poorly written extension can do, but it will make extensions more robust to version upgrades of Firefox.