To not let people know if you’ve read their Facebook chat messages, go to this website and press the green Get Chat Undetected button. It will then detect what web browser you’re using.
If you’re using Google Chrome, a Chat Undetected installation file will be downloaded into your computer. Close Google Chrome, run that installation file and you’re done.

If you’re using Mozilla Firefox, an add-on notice will appear confirming the installation of Chat-Undetected. Click on Install Now to complete the installation.
Now you’re all set, the irritating ‘Seen’ status is now gone and the friends you are chatting with will not be able to tell if you have ‘seen’ their message.