How to add a custom filter in AdBlock?
how to Turn Off Seen facebook Messages Chat?
you should have adblock or adblock plus addon or extensions for your browser(if you dont have then search on google and install it)
after that go to adblock icon
custom filter -~add filter group -~show/hide filter
then add two new filters
thats all now sender cant able to seen that you read his or her message 🙂
How To Hide Seen On Facebook Read Chat Messages,turn off the SEEN feature,Prevent Knowing When You’ve Seen a Message,how to use Adblock Plus custom filters