Look like? In what way does the interface of a browser changing make it more challenging to support as a website? Firefox looks a bit different now to a year ag

Look like? In what way does the interface of a browser changing make it more challenging to support as a website? Firefox looks a bit different now to a year ago with Quantum etc, and add-ons are very different, but I can't think of a single website that would be affected by those changes. Can you elaborate on what you're getting at here?