The way I see it, offline isn't primarily designed for websites, it's to replace apps that you'd install on you phone.

I agree on the installation confirmation - you have to agree before installing an app and the whole point of the blog post is that the line between a native app and a service worker site is getting smaller. So a similar level of permission should be requested before installing something locally.

In firefox (about:config):

  dom.serviceWorkers.enabled = false