This script allows you not to send a "read-message" status to your friends, by blocking every request to http(s)://www.facebook.com/ajax/mercury/change_read_status.php.
- Firefox: setup GreaseMonkey, and go to this link
- Chrome: go to this link
Supported browsers: Firefox & Chrome (Greasemonkey script). Please do not hesitate to make a pull request to support IE !