This is the plugin you’ve been waiting for, to keep your Facebook friends from seeing that you read what they said on Facebook in the chat column. If you look below this sentence you’ll see it in action in Google Chrome:
You just click the “Install” button if you are viewing the add-on page from Google Chrome and it will install automatically, no mumbo jumbo required, though you may need to restart Chrome to get it to show up in your browser. You can switch it on or off at any time, right from the top right of your Chrome browser. If you are using Firefox, check out “Unseen” here.