Indeed, I configured a Redirector[1] rule to do it:

    Redirect:     https?://www\.amazon\.com/(.*)
    Pattern type: Regular Expression
    to:           https://smile.amazon.com/$1
    Applies to:   Main window (address bar)

[1]: https://addons.mozilla.org/en-US/firefox/addon/redirector/

It does: https://www.amazon.com/b?ie=UTF8&node=15576745011

But then you need a browser extension just for that? It’s a bit overkill. I wonder if this works well with /etc/hosts or local DNS.

So? The amount of additional memory, disk, and processing it takes up are basically nothing in the greater scheme of things.

Smile is available on the iOS Amazon mobile app, but only if you allow Amazon to send you promotional notifications.