This userscript blocks all POSTS to change_read_status.php, delivery_receipts.php, and typ.php. As a result, other people on Facebook chat will not be able to see when you are typing or when you've opened a message.
This is a fork of Loïs Di Qual's original userscript here, adding an additional typing block and blocking POSTs to the new delivery_receipts.php. All credit to him! :3
This is a userscript, so it requires GreaseMonkey on Firefox or TamperMonkey on Chrome.
Once you've installed one of those addons, install this script by going here.