Finally, here's a working tutorial to disable the SEEN message from appearing on Facebook chat. It was easier earlier to pretend as if you were offline, if someone you don't like to chat with started a conversation. But after the SEEN message started appearing, people don't usually do so. Your friends know that you have seen the message but still have not replied to it and that might not be the wisest thing to do.
Don't worry though, here's a solution to the problem to prevent your friends from knowing that you have already seen the message.
Follow this guide depending upon your browser. Google chrome users please scroll this page till you see For Google Chrome Users.
For Firefox Users :
First of all, download and install this add-on (AdBlock Plus) on your browser.
Now, look on the bottom of your browser. There should be a logo (like the own shown on the picture below) spelling ABP. Click on it and select Filter Preferences...
Go to Custom Filters tab and click on Add filter group button. Name it anything you like; I've entered Facebook.
Click on the Actions button and select Show/hide filters. You will see a new pane opening right beside it.
Click on Add Filter button on the pane and paste this :
Click on Close.
Now onwards, even if you see the chat message, your friends will not receive the SEEN text on their chat box, hence they won't know that you have decided not to reply.
Also, if you don't want your friends to see that you are typing while on chat box, then just add this filter :
For Google Chrome Users:
Just download and install this add-on which will do the job for you. No need to paste anything anywhere.
PS. This won't work if you open Full Conversation and chat with your friend from there.
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