Earlier quoted context omitted.
Creating XUL, really? You cannot create an restricted API for everything, the very point of extensions, plugins, addons is community is always bigger than your developers team, and much, much more diverse. The 'long tail' effect is what extensions community is about, not '90 per cent of users'. 90 per cent of users might need just a couple of the same extensions, the other 10 per cent of users need a thousand of diff…
You can't create a UI that makes everyone happy either. For example, extensions that use the tabs WebExtension API are significantly more varied than the removed Panorama feature.
That's why every software project having a user community has theme support.