clear-facebook-notifications-once.user.js

// ==UserScript==
// @name Clear Facebook Notifications Once
// @description If you have multiple Facebook tabs open
// @match https://*.facebook.com/*
// @namespace https://github.com/johan/
// ==/UserScript==

const onStorageChange, onNotificationClick;

window.addEventListener('storage', onStorageChange = (e) => {
  if (e.key !== 'clearNotification') return;
  let v = e.newValue;
  let n = document.querySelector(`.jewelItemNew[data-alert-id="${v}"]`);
  if (n) n.classList.remove('jewelItemNew');
  console.log(n ? 'Cleared notification' : 'Missing notification', v);
});

document.addEventListener('click', onNotificationClick = (e) => {
  let {target} = e;
  let id;
  while (target) {
    if ((id = target.dataset.alertId)) {
      localStorage.clearNotification = id;
      console.log('broadcast clearNotification', id);
      break;
    }
    target = target.parentNode;
  }
});