Padding is typically part of the hitbox, margin is not. What you are describing sounds like margin to me. Especially for Firefox since their UI is made with web tech.

But besides being overly pedantic I agree with your general point. If their is a small gap between hitbox and edge of the screen it is very annoying.

But the worst thing IMO is Windows 11 windows which you can grab by the titlebar but not the content area. However often there is no visual indication of where the titlebar starts/stops.