This is the source to bootstrap addon "Custom About Page Demo". It demonstrates how to create an about:**** url for the MDN document located here: https://developer.mozilla.org/en-US/docs/Custom_about:_URLs#For_Firefox_4_%28Second_Approach%29

bootstrap.js

const {classes: Cc, interfaces: Ci, utils: Cu, results: Cr, manager: Cm} = Components;
var selfPath; //note: file system on windows uses \ (forward slash)) but firefox browser uses / (backward slash) //do not edit //the jar will have forward slashes but to navigate so if just want a file within the main folder its selfPath + fileName. but if want folder its selfPath + fileName + '\\'fileInFolder //so as u can see must use forward slashes to navigate folders

Cu.import('resource://gre/modules/XPCOMUtils.jsm');
Cu.import('resource://gre/modules/Services.jsm');
Cm.QueryInterface(Ci.nsIComponentRegistrar);

var AboutModuleUnloaders = [];
function AboutModule() {}

function registerAbout() {
	AboutModule.prototype = {
		uri: Services.io.newURI(selfPath + 'frontend module/index.txt', null, null), //EXAMPLE1: uri: Services.io.newURI('http://www.bing.com/', null, null), //EXAMPLE2: uri: Services.io.newURI('chrome://about-addons-memory/content/about.xhtml', null, null),
		classDescription: 'This shows custom about page demo',
		classID: Components.ID('9acb8b00-8616-11e3-baa7-0800200c9a66'), //EXAMPLE: classID: Components.ID('1704E6F0-8039-11E3-9CE1-C4766188709B'),
		contractID: '@mozilla.org/network/protocol/about;1?what=demo', //EXAMPLE: contractID: '@mozilla.org/network/protocol/about;1?what=yabba'
		QueryInterface: XPCOMUtils.generateQI([Ci.nsIAboutModule]),
		newChannel: function (aURI) {
			let chan = Services.io.newChannelFromURI(this.uri);
			chan.originalURI = aURI;
			return chan;
		},
		getURIFlags: function (aURI) 0
	};
	for (let[y, cls] in Iterator([AboutModule])) {
	   //Cu.reportError('y: ' + y);
	   //Cu.reportError('cls: ' + cls);
		try {
			var factory = {
				_cls: cls,
				createInstance: function (outer, iid) {
					if (outer) {
						throw Cr.NS_ERROR_NO_AGGREGATION;
					}
					return new cls();
				}
			};
			Cm.registerFactory(cls.prototype.classID, cls.prototype.classDescription, cls.prototype.contractID, factory);
			AboutModuleUnloaders.push(function(){
			 Cm.unregisterFactory(factory._cls.prototype.classID, factory);
			});
		} catch (ex) {
			Cu.reportError('failed to register module: ' + cls.name + '\nexception thrown: ' + ex);
		}
	}
}

function unregisterAbout() {
	for (var i=0; i<AboutModuleUnloaders.length; i++) {
		AboutModuleUnloaders[i]();
	}
}



function startup(aData, aReason) {
	Cu.reportError('startup');
	selfPath = aData.resourceURI.spec; //has final slash at end so for use use as: "aData.resourceURI.spec + 'bootstrap.js'" this gets the bootstrap file //note the final slash being a backward "/" is very important
	Cu.reportError('selfPath = "' + selfPath + '"');
	registerAbout();
	Cu.reportError('about registered')
}

function shutdown(aData, aReason) {
	if (aReason == APP_SHUTDOWN) return;
	
	unregisterAbout();
	
}

function install() {}

function uninstall() {}

frontend module\index.txt

this is the frontend

install.rdf

<?xml version="1.0" encoding="utf-8"?>
	<!-- This Source Code Form is subject to the terms of the Mozilla Public
   - License, v. 2.0. If a copy of the MPL was not distributed with this
   - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
	<RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:em="http://www.mozilla.org/2004/em-rdf#">
	  <Description about="urn:mozilla:install-manifest">
		<em:id>custom-about-page-demo@jetpack</em:id>
		<em:version>initial</em:version>
		<em:type>2</em:type>
		<em:bootstrap>true</em:bootstrap>
		<em:unpack>false</em:unpack>

		<!-- Firefox -->
		<em:targetApplication>
		  <Description>
			<em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
			<em:minVersion>7.0</em:minVersion>
			<em:maxVersion>10.0a1</em:maxVersion>
		  </Description>
		</em:targetApplication>

		<!-- Front End MetaData -->
		<em:name>custom about page demo</em:name>
		<em:description>This addon shows how to make a custom about page</em:description>
		<em:creator>Noitidart</em:creator>
		<em:iconURL/>
		<em:icon64URL/>
		<em:optionsType>2</em:optionsType>
	  </Description>
	</RDF>

Gists do not support folders it looks like. If you would like to try this addon, in the bootstrap.js directory, create a folder called "frontend module" and then inside it place index.txt.