Maybe Firefox needs to add a new "Clipboard access" permission that can be granted on a site-per-site basis. When disabled, simple highlight and copy could still be enabled if hidden text cannot be added in between normal text.
The same permission model could be used system wide, but I do not think that such a feature exists on the X server or Wayland. Maybe using a wrapper that runs before the Desktop Environment?
It was already like this in Europe when I began to use Spotify in 2015. I do not hate it because the app's free tier is already unusable to me due to the adverts.