Need to leave a server without leaving the IRC client you are using? Well, the command '/quit' is perfect for the job. The command works in all IRC clients, and removes you from the server. If you want, you can even send a reason.
In Mibbit, if you quit without a reason, the default reason is 'http://www.mibbit.com ajax IRC Client', though you can have your own reason with '/quit Your Reason Here'. This command works on both the widget and the main client. In the past, it did not work on widgets.