window.onload works in Firefox+Greasemonkey script but not in a Chrome userscript?

The reason that exampleFunction was undefined is because Chrome userscripts operate in a sandbox ("isolated world"). Note that Greasemonkey scripts often operate in a sandbox too, but yours is currently running with an implicit @grant none.
If your script were to use a GM_ function, it would stop working in Firefox too.

To make this script work on both browsers (and a few others, as well), use Script Injection similar to this answer.

However, there is another hitch, since that script is using window.onload. Chrome userscripts, with the default execution start-mode, will often never see the onload event.

To get around that, add // @run-at document-end to the metadata block.

So the script becomes:

// ==UserScript==// @name            SomeName// @namespace       http://example.com/userscripts// @description     Greets the world// @include         http://example.com/*// @run-at          document-end// @grant           none// ==/UserScript==function GM_main () {    window.onload = function () {        console.log(exampleFunction);        alert("LOADED!");    }}addJS_Node (null, null, GM_main);//-- This is a standard-ish utility function:function addJS_Node (text, s_URL, funcToRun, runOnLoad) {    var D                                   = document;    var scriptNode                          = D.createElement ('script');    if (runOnLoad) {        scriptNode.addEventListener ("load", runOnLoad, false);    }    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);}

If you want the equivalent to onLoad, which doesn't fire till all images on the page are loaded, you want to use // @run-at document-idle in your metadata block. The default, document-end, fires when the DOM is loaded, the equivalent of document.ready.

Have you tried calling examplefunction with brackets ? :)Like this:

console.log(exampleFunction());

If you try it in chrome console, you have to add brackets to call function.