The add-on works correctly (tested under Firefox 54) and will be without a doubt an useful tool against at least one of the myriad of annoyances found in how Facebook works.
The one hiccup I found, that is up to each user to think of as a problem or not, is one that is shared with similar working add-ons (those preventing Facebook from notifiying others that we have seen their messages): These types of extensions always mark all such messages as "unread" by you in your own account. So you always log-in to discover a notification on your FB bar of handfuls of messages waiting to be seen. Is as if not letting the sender know you've seen the message means not actually seeing it.
I think your feelings towards the small gripe I just described will be decisive on weather you like this extension or not.