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.
Workaround: Disable JavaScript.