This bootstrap addon shows how to add a menu item to the content context menu. FORK: This fork is different in that it shows how to make that menu item hidden or shown based on if the user opens the popup from a link that contains 'mozilla' in its href.

bootstrap.js

const {interfaces: Ci,utils: Cu} = Components;
Cu.import('resource://gre/modules/Services.jsm');

/*start - windowlistener*/
var windowListener = {
	//DO NOT EDIT HERE
	onOpenWindow: function (aXULWindow) {
		// Wait for the window to finish loading
		let aDOMWindow = aXULWindow.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowInternal || Ci.nsIDOMWindow);
		aDOMWindow.addEventListener("load", function () {
			aDOMWindow.removeEventListener("load", arguments.callee, false);
			windowListener.loadIntoWindow(aDOMWindow, aXULWindow);
		}, false);
	},
	onCloseWindow: function (aXULWindow) {},
	onWindowTitleChange: function (aXULWindow, aNewTitle) {},
	register: function () {
		// Load into any existing windows
		let XULWindows = Services.wm.getXULWindowEnumerator(null);
		while (XULWindows.hasMoreElements()) {
			let aXULWindow = XULWindows.getNext();
			let aDOMWindow = aXULWindow.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowInternal || Ci.nsIDOMWindow);
			windowListener.loadIntoWindow(aDOMWindow, aXULWindow);
		}
		// Listen to new windows
		Services.wm.addListener(windowListener);
	},
	unregister: function () {
		// Unload from any existing windows
		let XULWindows = Services.wm.getXULWindowEnumerator(null);
		while (XULWindows.hasMoreElements()) {
			let aXULWindow = XULWindows.getNext();
			let aDOMWindow = aXULWindow.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowInternal || Ci.nsIDOMWindow);
			windowListener.unloadFromWindow(aDOMWindow, aXULWindow);
		}
		//Stop listening so future added windows dont get this attached
		Services.wm.removeListener(windowListener);
	},
	//END - DO NOT EDIT HERE
	loadIntoWindow: function (aDOMWindow, aXULWindow) {
		if (!aDOMWindow) {
			return;
		}
		var contentAreaContextMenu = aDOMWindow.document.getElementById('contentAreaContextMenu');
		if (contentAreaContextMenu) {
			var menuItem = aDOMWindow.document.createElement('menuitem');
			menuItem.setAttribute('label', 'You clicked on a link that contains mozilla in the href');
			menuItem.setAttribute('oncommand', 'alert("hi")');
			menuItem.setAttribute('id', 'myMenuItem');
			menuItem.setAttribute('hidden', 'true');
			contentAreaContextMenu.appendChild(menuItem);
			
			contentAreaContextMenu.addEventListener('popupshowing',decideToShowMyMenuItem,false);
		}
	},
	unloadFromWindow: function (aDOMWindow, aXULWindow) {
		if (!aDOMWindow) {
			return;
		}
		var contentAreaContextMenu = aDOMWindow.document.getElementById('contentAreaContextMenu');
		if (contentAreaContextMenu) {
			var myMenuItem = aDOMWindow.document.getElementById('myMenuItem');
			contentAreaContextMenu.removeChild(myMenuItem);
			contentAreaContextMenu.removeEventListener('popupshowing',decideToShowMyMenuItem,false);
		}
	}
};
/*end - windowlistener*/

function decideToShowMyMenuItem(event) {
	var window = event.target.ownerDocument.defaultView;
	var document = window.document;
	var myMenuItem = document.getElementById('myMenuItem');
	
	if (myMenuItem) {
		var popupNode = document.popupNode; //this is the target clicked on but its prefered to use triggerNode; //note: very different from event.target //see here: https://developer.mozilla.org/en-US/docs/Web/API/document.popupNode?redirectlocale=en-US&redirectslug=DOM%2Fdocument.popupNode //event.target is the menupopup itself

		if (popupNode.parentNode.nodeName == 'A') {
			//sometimes people put elements between the a tags like: <a href="blah"><b>bolded link</b></a>
			//so we check to see if parentNode is link
			popupNode = popupNode.parentNode;			
		}
		//cDump(popupNode);
		var href = popupNode.href; //dont do popupNode.getAttribute() because if href is relative path, it wont have base name in it
		Cu.reportError('href = ' + href)
		if (href) { //test if clicked on an element that has href attribute, or you can do popupNode.nodeName == 'A' to test if clicked on link
			if (href.toLowerCase().indexOf('mozilla') > -1) {
				myMenuItem.hidden = false; //show the menu item
			} else {
				myMenuItem.hidden = true; //keep the menu item hidden because 
			}
		}
	}
}

function startup(aData, aReason) {
	windowListener.register();
}

function shutdown(aData, aReason) {
	if (aReason == APP_SHUTDOWN) return;
	windowListener.unregister();
}

function install() {}

function uninstall() {}

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>demo-contextmenu-menuitem-shown-when-link-with-mozilla@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>demo contextmenu menuitem shown when link with mozilla</em:name>
		<em:description>This bootstrap addon shows how to add a menu item to the content context menu. FORK: This fork is different in that it shows how to make that menu item hidden or shown based on if the user opens the popup from a link that contains 'mozilla' in its href.</em:description>
		<em:creator>Noitidart</em:creator>
		<em:iconURL/>
		<em:icon64URL/>
		<em:optionsType>2</em:optionsType>
	  </Description>
	</RDF>

This is a copy paste bootstrap addon that shows how to add menu items to the content area context menu and only show the menu item if the user opened popup menu from a link that contains the word mozilla.

To change this behavior edit the decideToShowMyMenuItem function.

Edit in loadIntoWindow and unloadFromWindow to add more menu items.