An MOTD is also known as a "Message of the Day". When a user connects to a server, it sends the message. You can find the irc.mibbit.net MOTD here.
In the main client, users can turn it off in preferences. It is in the "What to Show" section of "Chat Data". (Currently staging only).
In the widget, users can turn it off with the URI parameter "NoServerMotd=true".