But since Facebook activated this feature, you can not ignore chat messages on Facebook for your chat friends will know that you've seen messages he sent. On Facebook there is no setting that can disable this feature, but now there is an add-on application for browser which can turn off this feature, here are the steps... Open this website, click the green button labeled "get chat undetected". Chat Undetected lets you disable Facebook Messenger's read receipt feature, preventing others from seeing if you have viewed a message or not. If your browser is Firefox, then the form of this application is a Firefox add-on, you can install it directly.
If you use chrome, after you click on the green button, chrome will download a small exe file. Close chrome and then install the application. Once the add-on is installed, chat friends can not recognize whether we've seen the messages he sent or not.
How do I uninstall?
If you want to uninstall or to disable the add-on for a while, you can just go to the add-on browser setting then select disable or uninstall. I am sure this add-on can help you maintain your privacy when dealing with chatting on Facebook.