While preventing firefox from connecting to IRC servers is one way to solve the problem, the question is "how?".
IRC connections are usually silent until the client sends a username and a nickname, and they have both been processed, making it next to impossible to detect if the server at the other end is an IRC server until everything has already been sent down the tube.
Port filtering is useless, as IRC servers can be run on any port.
This is more a vulnerability/issue in the IRC server implementation used on freenode, as it doesn't reject clients who send junk along when initialising the connection (the HTTP headers), and would be far, far easier to fix in the server.
After all, this isn't a firefox specific issue. Concievably, you could do this with anything that uses a TCP stream, from an IMAP client to, as the article said, SIP.