Rated 3 out of 5 stars

PROS:
- Does the job it's supposed to do.

CONS:
- No "on the fly" button to turn it on/off like in the Google Chrome version.
- Works by blocking the facebook php script which sends the "seen" notification. Problem is that same script also marks the messages as "read". This means that all new messages will forever be marked as "unread" as long as this add-on is enabled (thus, lack of on/off button is a real problem).

A better alternative is to use this user-script for Greasemonkey/Scriptish:
https://github.com/ldiqual/facebook-read-status-disabler/raw/master/facebook-read-status-disabler.user.js

That way you can easily turn it on/off on the fly!