Did this start recently and for all Thunderbird clients at once, or perhaps after Thunderbird updated? You could always insall an older Thunderbird version to see if that works to rule out the last. If this does not apply, could there be some company security software involved? For instance, Symantec IM Manager is able to block IRC messages while allowing logins, and perhaps it’s part of Symantec Endpoint Protection? Try disabling that and perhaps connecting to a non-corporate network such as your home network if possible for a test. I don’t think it should be a port (to connect to) issue.