userscript - Add item to right-click menu

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));
  }
}

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);
})();