After clicking the following link, the log contains a "Service Worker Ready" message.
After clicking the following link, the "navigator.serviceWorker.ready" promise never gets resolved.
After clicking the following link, the browser freezes the page.
You can try granting or denying the permission to cover more cases. To reset permission to default, use the button left to the URL, for Receive Notifications select Always Ask, and reload.
If not freezing Firefox, reading the subscription would return null, unless you subscribe to push notifications. (Needs a registered service worker and notification permission.)
Prior to registering another service worker, you must unregister the previous one.