Hi,

Firefox 2.0.0.18 fixes a cross-domain theft of image data. Firefox 3
unaffected. It's another interesting case where a redirector confuses
the browser about the true origin of a piece of content. If evil.org
hosts a redirector, e.g. evil.org/redir, and an image is loaded via
this redirector, the image will be treated as a same-domain image. In
this event, the image pixel data may easily be stolen by rendering the
image to a canvas and using the getImageData() JavaScript API.

Advisory: http://scary.beasts.org/security/CESA-2008-009.html 

Blog post: http://scarybeastsecurity.blogspot.com/2008/11/firefox-cross-domain-image-theft-and.html 

Cheers
Chris