- Create a bookmark with the following text in the URL field:
javascript:(function() {
var links = document.querySelectorAll('link[rel*=icon]');
var href = prompt('Enter your new shelf icon\'s URL');
href && links.forEach((link) => link.href = href);
})();
- Click the bookmark on your desired page
- Enter the desired shelf icon URL and click OK.
- Add the page to your shelf.
Thanks to /u/dimethylarginine for discovering this trick.