A Mozilla Foundation Security Advisory reports:
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. The "firescrolling" example demonstrates arbitrary code execution (in this case downloading a file) by convincing the user to scroll twice.