This is a big deal: Firefox extension makes social network ID spoofing trivial.
It would, I gather, be a serious violation of the University of Miami acceptable use policies for me to test drive this at work, so I can't report on how it does. But if anyone has tried this out somewhere else, please let me know in the comments if it works as advertised.
The author of the article optimistically suggests that having something like this in the wild will cause a revolution in site security. That may work at the very highest profile sites like Facebook, but overall, that's just a dream, unfortunately.
I just hope that using a VPN protects against this. (I think it will.)
(Thanks to TB for putting me on to this.)