https://addons.mozilla.org/en-US/firefox/addon/paintitgreyscale/

paintitgreyscale.js

// ==UserScript==
// @name          Paint It Greyscale
// @namespace     John Wrenn
// @description    Surf the web in soothing shades of grey!
// @author        shadowbq
// @grant         none
// @include       *
// ==/UserScript==

var $;


//document.body.style.visibility = 'hidden';

// Add jQuery
(function(){

	if (typeof unsafeWindow.jQuery == 'undefined') {
		var GM_Head = document.getElementsByTagName('head')[0] || document.documentElement,
			GM_JQ = document.createElement('script');

		GM_JQ.src = 'http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js';
		GM_JQ.type = 'text/javascript';
		GM_JQ.async = true;

		GM_Head.insertBefore(GM_JQ, GM_Head.firstChild);
	}
	GM_wait();
	
})();

// Check if jQuery's loaded
function GM_wait() {
	if (typeof unsafeWindow.jQuery == 'undefined') {
		window.setTimeout(GM_wait, 100);
	} else {
		$ = unsafeWindow.jQuery.noConflict(true);
		letsJQuery();
	}
}

// All your GM code must be inside this function
function letsJQuery() {

	var ns = "http://www.w3.org/2000/svg";
	var svg = document.createElementNS(ns, "svg");
	var filter = document.createElementNS(ns, "filter");
	var matrix = document.createElementNS(ns, "feColorMatrix");
	var values = "0.3333 0.3333 0.3333 0 0 0.3333 0.3333 0.3333 0 0 0.3333 0.3333 0.3333 0 0 0 0 0 1 0";

	filter.setAttribute('id','fltr');
	matrix.setAttribute('values',values);

	filter.appendChild(matrix);
	svg.appendChild(filter);
	
	$(document).ready(function() {

	  document.body.appendChild(svg);
	  document.body.style.filter="url(#fltr)";

	});
	

}

Grease Monkey Compiler:

https://arantius.com/misc/greasemonkey/script-compiler.php

GUID for plugin: ca5a8350-c231-462b-a0b9-736de6a028e3

Dumb question: How do I get this to run? I installed it as an Addon and restarted Firefox, but could not find a menu item, button, etc. to turn on the effect. Then, since I already had Greasemonkey for something else, I put your code found here into a new User Script, but that didn't do anything either.