Fayte sent me this Bindshell link today that discusses a way to bypass port blocking in Firefox, Opera and Konqueror. The obvious implications for this attack are to circumvent the restrictions built into Firefox in particular. This restriction makes it hard to do things like attack non HTTP ports due to a restriction build into default versions of Firefox. That really does put a crimp into port scanning, but this is a clever way to circumvent it using a malicious FTP server.
The basic concept is if you send a header like so: 227 Entering Passive Mode (192,168,0,1,84,149) your browser will be redirected if it supports the PASV command (which Firefox, Opera and Konqueror do). Very clever implementation and nice work from Mark at Bindshell!