Whenever you have a chance to do something in a declarative way, it's the better way to go IMO. Sad to see XUL removed... newer Firefox extensions are integrated much more poorly and are slower when the browser is busy (due to JS). Probably not noticeable for one tab though.
If Firefox wanted they could have created a new extension system with XUL that would also have the same limitations the web extensions do. For fairly obvious reasons they decided to transition to Web tech based extensions and used that opportunity to tighten up security as well.