Plugins can be used to load privileged content — Mozilla

Plugins (such as flash) can be used to load privileged content into a frame. Once loaded various spoofs can be applied to get the user to interact with the privileged content. Michael Krax’s “Fireflashing” example demonstrates that an attacker can open about:config in a frame, hide it with an opacity setting, and if the attacker can get the victim to click at a particular spot (design some kind of simple game) you could toggle boolean preferences, some of which would make further attacks easier.