dg3420td_username_field_editable.user.js

// ==UserScript==
// @name         Make YouSee Modem (Kaon DG3420TD) Web Interface Username Field Editable
// @version      1.0
// @description  Removes the readonly attribute from the router username input field, allowing you to log in as other users. Probably works for Orange Livebox, Swisscom and other devices running SoftAtHome firmware.
// @author       narkopolo
// @match        *://192.168.1.1/*
// @require  http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js
// @require  https://gist.github.com/raw/2625891/waitForKeyElements.js
// ==/UserScript==

var usernameField = "div.sah_dialog_body_login input#login_username";

waitForKeyElements (
    usernameField,
    removeReadOnly
);

function removeReadOnly (jNode) {
    console.log("Trying to remove readonly attr");
    document.querySelector(usernameField).removeAttribute('readonly');
}