userscript - Add item to right-click menu - firefox

Question

I am writing a userscript for Greasemonkey and want to add an item to right click context menu. If I'm not mistaken, this function is called window.addEventListener("contextmenu", ...).
Can you give me example of this?

Answer 1

Following script was copied from http://userscripts-mirror.org/scripts/review/151097:
Important warning! This copy is unchanged and untested. There might be security issues with userscripts. GM is very powerful and if used on the wrong page the safety of your computer may be at risk.
/*
Google Image Search Context Menu
Add 'Search by Image' in browser context menu when you
right click on image to search Google with that image.
Copyright (C) 2012 LouCypher
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>
*/
// ==UserScript==
// #name Google Image Search Context Menu
// #namespace http://userscripts.org/users/12
// #description Add 'Search by Image' in browser context menu when you right click on image to search Google with that image.
// #version 1.2
// #author LouCypher
// #license GPL
// #resource license https://raw.github.com/LouCypher/userscripts/master/licenses/GPL/LICENSE.txt
// #updateURL https://userscripts.org/scripts/source/151097.meta.js
// #include *
// #exclude file://*
// #grant GM_openInTab
// ==/UserScript==
if (!("contextMenu" in document.documentElement &&
"HTMLMenuItemElement" in window)) return;
var body = document.body;
body.addEventListener("contextmenu", initMenu, false);
var menu = body.appendChild(document.createElement("menu"));
menu.outerHTML = '<menu id="userscript-search-by-image" type="context">\
<menuitem label="Search Google with this image"\
icon="data:image/png;base64,\
iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAABHNCSVQICAgIfAhkiAAAAAlwSFlz\
AAAK6wAACusBgosNWgAAABx0RVh0U29mdHdhcmUAQWRvYmUgRmlyZXdvcmtzIENTNXG14zYAAAEl\
SURBVDiNY/z//z8DJYCRkIKsthv/kRX9Z2BgmFalARdiIcaGKZXqcH5O+01U+ay2G3MYGBiSiXUm\
mofnsBDSjEUTMkiBe2Eq1JnZ7TcZBHhZGNythBl0lLkZODmYGX7++sdw/sZnhl3H3zF8+voHwwsY\
FkR5ijNICLMzTF31hOHnr38MHGxMDJlhMgwv3vxkWL7jJYpaJmzu0lTigWtmYGBg+PHrH8P0VU8Y\
tJV5MNRiNYCfmxmuGQZ+/PrHwMmOqRyrAX///WfgYEOV4mBjwjAUpwHHL31iyA6XgRvCwcbEkBUm\
w3DuxmcMtVgDkYONicHLVoTBSJOXgYONieHHz38Ml+98Ydh88DXDtx//CBtACmBiYGCYS4H+OYyU\
5kasgUgKAADN8WLFzlj9rgAAAABJRU5ErkJggg=="></menuitem>\
</menu>';
document.querySelector("#userscript-search-by-image menuitem")
.addEventListener("click", searchImage, false);
function initMenu(aEvent) {
// Executed when user right click on web page body
// aEvent.target is the element you right click on
var node = aEvent.target;
var item = document.querySelector("#userscript-search-by-image menuitem");
if (node.localName == "img") {
body.setAttribute("contextmenu", "userscript-search-by-image");
item.setAttribute("imageURL", node.src);
} else {
body.removeAttribute("contextmenu");
item.removeAttribute("imageURL");
}
}
function addParamsToForm(aForm, aKey, aValue) {
var hiddenField = document.createElement("input");
hiddenField.setAttribute("type", "hidden");
hiddenField.setAttribute("name", aKey);
hiddenField.setAttribute("value", aValue);
aForm.appendChild(hiddenField);
}
function searchImage(aEvent) {
// Executed when user click on menuitem
// aEvent.target is the <menuitem> element
var imageURL = aEvent.target.getAttribute("imageURL");
if (imageURL.indexOf("data:") == 0) {
var base64Offset = imageURL.indexOf(",");
if (base64Offset != -1) {
var inlineImage = imageURL.substring(base64Offset + 1)
.replace(/\+/g, "-")
.replace(/\//g, "_")
.replace(/\./g, "=");
var form = document.createElement("form");
form.setAttribute("method", "POST");
form.setAttribute("action", "//www.google.com/searchbyimage/upload");
form.setAttribute("enctype", "multipart/form-data");
form.setAttribute("target", "_blank");
addParamsToForm(form, "image_content", inlineImage);
addParamsToForm(form, "filename", "");
addParamsToForm(form, "image_url", "");
body.appendChild(form);
form.submit();
}
} else {
GM_openInTab("https://www.google.com/searchbyimage?image_url=" +
encodeURIComponent(imageURL));
}
}

Answer 2

Unfortunately the <menu>+<menuitem> solution no longer works since Firefox 85 (and never worked in other browsers I believe) see here and here.
Below is an example of how to add an item to the right-click menu (tested in Tampermonkey & Firefox 91) although sadly it shows up under a 'Tampermonkey' sub-menu rather than as a top-level item in its own right.
The key part is #run-at context-menu
// ==UserScript==
// #name Name of context menu item
// #include *
// #version 0.1
// #grant GM_openInTab
// #run-at context-menu
// ==/UserScript==
(function() {
'use strict';
// If you want to do something based on the selected text
var selectedText = window.getSelection().toString();
// Construct your destination URL
var url = ...
// Open in new, foreground tab
GM_openInTab(url, false);
})();