this hasn't been working for a long time. Try this other firefox extension instead:

https://addons.mozilla.org/en-US/firefox/addon/ad-blocker-for-facebook-app/

Or get the chrome Facebook ad blocker at:

https://chrome.google.com/webstore/detail/video-ad-blocker-for-face/cfelggdancjlfmekcfekjemffindioac