When you log on to yahoo chat, my local firewall started complaining that Firefox is about to connect to some yahoo hosts on strange ports:
Doing a network trace showed the same – Firefox is connecting to these servers directly and is not willing to comply with my proxy settings – nasty.
I then blocked all traffic but to my webgateway on port 9090. Then a simply URL Filtering rule for Instant Messaging as category stops it, once being forced, the thing posts to a site known in the IM category.n your fire
POST http://188.8.131.52/ HTTP/1.1
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:184.108.40.206) Gecko/20100722 Firefox/3.6.8
You can try blocking the port above in your firewall, or use a local desktop firewall to block these ports for clients.