Simple Alternative to Greasemonkey

There is no simpler alternative to Firefox's Greasemonkey or to Chrome's userscripts that runs user JS automatically. You could write your own extension/add-on, but there wouldn't be much point to it.

If you don't care about the awesome extra power that GM and userscripts provide and always want to just "(take) a few lines of JS and (run) them after page load for a certain site" -- ignoring iframes, then just use the following code as a base-template for all of your scripts:

// ==UserScript==// @name    _Base template for simple, cross-browser, JS injection.// @match   *://YOUR_SERVER.COM/YOUR_PATH/*// @run-at  document-start// ==/UserScript==if (window.top != window.self)  //-- Don't run on frames or iframes.    return;function scriptMain () {    // PUT ALL OF YOUR CODE HERE, INCLUDING ANY FUNCTIONS YOU CREATE.    console.log ("Hello World!");}window.addEventListener ("load", scriptMainLoader, false);function scriptMainLoader () {    addJS_Node (null, null, scriptMain);}function addJS_Node (text, s_URL, funcToRun) {    var D                                   = document;    var scriptNode                          = D.createElement ('script');    scriptNode.type                         = "text/javascript";    if (text)       scriptNode.textContent  = text;    if (s_URL)      scriptNode.src          = s_URL;    if (funcToRun)  scriptNode.textContent  = '(' + funcToRun.toString() + ')()';    var targ = D.getElementsByTagName ('head')[0] || D.body || D.documentElement;    targ.appendChild (scriptNode);}

Note that the @run-at document-start is required (for Chrome) but your code will still fire at document load.