Take privacy back into your own hands! Seen Blocker stops Facebook from reporting ‘Seen’ to messages that you open on Facebook.com. It also prevents the typing indicator from appearing as you type a message.
This extension is small and easy to use – two options are available which toggle the ‘Seen’ and ‘Typing’ blocking, and can be turned on and off at your leisure.
Note: This does NOT block the ‘Seen’ feature from Facebook group posts, only for messaging.
Source available on GitHub: https://github.com/MicahNus/Seen-Blocker