Restore the Modern, Red Unread Icon in Gmail using ViolentMonkey or TamperMonkey

blue-gmail-icon-be-gone.js

// ==UserScript==
// @name Gmail Count Red
// @namespace Violentmonkey Scripts
// @match *://mail.google.com/*
// @grant none
// ==/UserScript==

function fix (link) {
  const href = link.getAttribute('href');
  link.setAttribute('href', href.replace('-b', '_2x'));
}

const iconInterval = setInterval(() => {
  const link = document.querySelector('link[rel=icon]');

  if (link) {
    clearInterval(iconInterval);
    fix(link);

    const observer = new MutationObserver(((mutations) => {
      mutations.forEach((mutation) => {
        const nodes = mutation.addedNodes || [];
        const child = mutation.type === 'childList';
        if (child && nodes.length && nodes[0].nodeName === 'LINK') {
          fix(nodes[0])
        }
      });
    }));

    observer.observe(document.head, {
      attributes: true, 
      childList: true, 
      characterData: true 
    });
  }
}, 100);