@AC
In Firefox, it's apparently fairly trivial to detect if a user has a particular extension installed:
http://jeremiahgrossman.blogspot.com/2006/08/i-know-what-youve-got-firefox.html
Alternatively, it's presumably possible to detect if the DOM has been changed to hide an advert, and trigger a function to prevent loading of a video if this is the case (note: I don't know if this is how Adblock works, I'm not a Firefox extension developer).