One of the most annoying things about Facebook Messenger is that little timestamp that tells people when you've read messages, making it impossible to ignore someone and still read what they have to say. Now, after many years of it being there, someone's made a Chrome extension to get around it.
Facebook Unseen stops Messenger from sending the read receipts, so you can read through all off the messages you've been sent and the sender will be none the wiser. That makes it perfect for dealing with friends who expect you to reply right away, ignoring the person that keeps sending you mounds of nonsense everyday, or just because you feel that the time you read your messages is none of anyone else's business.
But if it's someone you are quite happy to talk to, you can manually mark the message as having been read for the sender to see. You don't want people noticing that you're responding before the read receipt shows up, do you? [Chrome Web Store]