F12 forever

// ==UserScript==
// @name         New Userscript
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        http://stackoverflow.com/questions/1155353/find-and-replace-in-a-webpage-using-javascript
// @grant        none
// ==/UserScript==

//var div = document.getElementById('answers-header');div.textContent = 'anything';
var div = document.getElementsByClassName('vote')[0]; div.innerHTML= "<div class=\"vote\">\r\n        <input type=\"hidden\" name=\"_id_\" value=\"1155353\">\r\n        <a class=\"vote-up-off\" title=\"This question shows research effort; it is useful and clear\">up vote<\/a>\r\n        <span itemprop=\"upvoteCount\" class=\"vote-count-post \">200<\/span>\r\n        <a class=\"vote-down-off\" title=\"This question does not show any research effort; it is unclear or not useful\">down vote<\/a>\r\n\r\n        <a class=\"star-off\" href=\"#\">favorite<\/a>\r\n        <div class=\"favoritecount\"><b>1<\/b><\/div>\r\n\r\n\r\n<\/div>";
//  [0] Is responsible for element position in page
//  to find by class you need use  div = document.getElementsByClassName('CLASS')[0];
// usetextContent instead innerHTML to change content in div
// то что ты написал, найдёт div с id='id' и внутри него создаст новый