Thanks for letting me know about it. I've updated the Add-on and tested again on Firefox (version 50.01). It should work now. You will get the update automatically if you keep the Add-on installed. Or, you can install it from here:
https://addons.mozilla.org/en-US/firefox/addon/facebook-no-ads-in-news-feed/versions/?page=1#version-0.0.8