Great addon. You can do this yourself by blocking a certain URL/announce etc at router/filter level but this makes it much simpler for people. Does exactly as described and blocks the report of you seeing message upon entering the message. However be warned as soon as you start typing it will update as seen so if you start but decide not to answer they will still know you have read message so only start if you're ready/available etc.
I like this product because I sometimes check my work account on a day off but don't want people to know and expect instant response. Only problem is that as it stands right now, I cannot tell if it is turned off or on?!? I used to be able to tell visually, but now both states look the same. If I can figure how to get it to show the difference again, I'll rate even better.