Because there is nothing Firefox can do to prevent it. I suppose Mozilla prefers they do it with a documented interface rather than modifying the Firefox instal

Because there is nothing Firefox can do to prevent it. I suppose Mozilla prefers they do it with a documented interface rather than modifying the Firefox install directly. If you want to prevent applications from installing plugins into each other, then you need an operating system that sandboxes each application (difficult), or use a package manager that is run by a trusted party (Linux).