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.
Also, more than 90% of the title bar are active UI elements like menus and close buttons, and you have to guess which few pixels are actually inert enough to allow dragging.
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.