disable notification seen on the Facebook chat.

  <recent weeks, Facebook has enabled the cat a new feature that lets you know if your friends saw the messages you've sent them, and it started to create conflicts with our friends.
there is a very effective solution to this problem
In fact it is an extension for Google Chrome and Mozilla Firefox, which now allows you to disable the notification "seen". Facebook is Unseen.
  For this, go to Google Chrome or Mozilla Firefox then download the extension
It remains for you to enable or disable the extension to prevent your friends know that you saw their message  .>