See https://github.com/getfider/fider/issues/530#issuecomment-451364591

fider-group-notifications.user.js

// ==UserScript==
// @name     Fider (ClassicPress petitions) improvements
// @version  2
// @grant    none
// @match    *://petitions.classicpress.net/notifications
// ==/UserScript==

function go() {
  const css = document.createElement('style');
  css.textContent = `
  #gm-mark-read {
    position: absolute;
    top: 140px;
    right: 30px;
    font-size: 125%;
  }

  #p-my-notifications .c-list .c-list-item {
    padding: 0;
    margin-bottom: 8px;
  }
  #p-my-notifications .c-list .c-list-item.adjacent {
    margin-bottom: 0;
  }
  #p-my-notifications .c-list .c-list-item a {
    display: flex;
  }
  #p-my-notifications .c-list .c-list-item a div.markdown-body {
    flex-grow: 1;
  }
  #p-my-notifications .c-list .c-list-item a span.info {
    flex-shrink: 0;
    align-self: center;
    margin-left: 8px;
  }
  `;
  document.head.appendChild(css);

  const button = document.createElement('button');
  button.id = 'gm-mark-read';
  button.textContent = 'Mark some as read';
  document.body.appendChild(button);
  button.addEventListener('click', promptAndMarkRead);

  let _originalNotifications = null;

  function getNotifications() {
    const notificationsList = document.querySelector('#p-my-notifications .c-list'); // note: first match only!
    if (!notificationsList) {
      return null;
    }
    if (!_originalNotifications) {
      _originalNotifications = Array.from(notificationsList.querySelectorAll('.c-list-item')).map(item => {
        const strongs = Array.from(item.querySelectorAll('strong'));
        const title = strongs[strongs.length - 1].innerText.trim();
        const url = item.querySelector('a').href;
        return {node: item, strongs, title, url};
      });
    }
    return Array.from(_originalNotifications);
  }

  function sortNotifications() {
    // Collect and process notification items
    const originalItems = getNotifications();
    if (!originalItems) {
      // Fider hasn't finished loading yet
      console.log('Waiting for Fider');
      setTimeout(sortNotifications, 300);
    }
    const itemsSorted = {};

    originalItems.forEach((item1, i) => {
      // Get title
      const strongs1 = item1.strongs;

      // Rearrange text to be more easily scannable
      strongs1[strongs1.length - 1].parentNode.insertBefore(
        strongs1[strongs1.length - 1],
        strongs1[strongs1.length - 1].parentNode.firstChild
      );
      if (strongs1.length > 1) {
        strongs1[0].parentNode.insertBefore(strongs1[0], null);
      }
      item1.node.querySelector('div.markdown-body p').childNodes.forEach(node => {
        if (node.nodeType === Node.TEXT_NODE) {
          switch (node.textContent.trim()) {
            case 'left a comment on':
              node.textContent = ' comment by ';
              break;
            case 'New post:':
              node.textContent = ' new post';
              break;
          }
        }
      });

      // Mark all notifications as read at once
      item1.node.querySelector('a').addEventListener('click', e => {
        e.preventDefault();
        const toMark = getNotifications()
        	.filter(n => n !== item1 && n.title === item1.title);
        markNotificationsRead(toMark, function(err) {
          if (err) {
            alert('An error occurred: ' + err.message);
          } else {
            document.location = item1.url;
          }
        });
      });

      // Sort notifications by petition title
      // Note: this is inefficient!
      if (!itemsSorted[item1.title]) {
        originalItems.slice(i + 1).forEach((item2, j) => {
          if (item2.title === item1.title) {
            item2.node.parentNode.insertBefore(item2.node, item1.node.nextSibling);
            item1.node.classList.add('adjacent');
            item1 = item2;
          }
        });
        itemsSorted[item1.title] = true;
      }
    });
  }
  window.addEventListener('load', sortNotifications);

  function markNotificationsRead(notifications, cb) {
    if (!notifications.length) {
      cb(null);
      return;
    }

    const url = notifications[0].url;

    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function() {
      if (xhr.readyState == XMLHttpRequest.DONE) {
        console.log(
          '%s: HTTP %d, %d bytes; %d left',
          url,
          xhr.status,
          xhr.responseText.length,
          notifications.length - 1,
          xhr.responseText
        );
        if (xhr.status === 200) {
          markNotificationsRead(notifications.slice(1), cb);
          notifications[0].node.parentNode.removeChild(notifications[0].node);
        } else {
          console.log(xhr.responseText);
          cb(new Error('HTTP ' + xhr.status));
        }
      }
    };
    xhr.open('GET', url);
    xhr.send();
  }

  function promptAndMarkRead() {
    const input = prompt('Mark all notifications read for posts matching:', '');

    const filter = new RegExp(input || '', 'i');
    const notifications = [];
    const notificationsByTitle = {};
    getNotifications().forEach(n => {
      if (filter.test(n.title)) {
        notifications.push(n);
        notificationsByTitle[n.title] = (notificationsByTitle[n.title] || 0) + 1;
      }
    });

    if (!input) {
      alert('No filter entered!\n\n'
        + 'Summary of all notifications:\n\n'
        + JSON.stringify(notificationsByTitle, null, 2)
      );
      console.log('Aborted!');
      return;
    }

    if (!notifications.length) {
      alert('No matching notifications!');
      return;
    }

    const ok = confirm(
      'Really mark ' + notifications.length + ' notification'
      + (notifications.length === 1 ? '' : 's') + ' as read?\n\n'
      + 'Counts by title: '
      + JSON.stringify(notificationsByTitle, null, 2)
    );
    if (!ok) {
      console.log('Aborted!');
      return;
    }

    markNotificationsRead(notifications, function(err) {
      if (err) {
        alert('An error occurred: ' + err.message);
      }
    });
  }
}

const script = document.createElement('script');
script.textContent = go.toString() + '; go();';
document.head.appendChild(script);
//go();