Can a site detect if I use AdBlock in Firefox?

Can a site detect if I use AdBlock in Firefox? I recently visited (an official) website in search for an episode of a TV-show. The site popped an information dialog stating that I had to disable AdBlock to see the content. Is this at all possible or is it just an educated guess? Does Firefox advertise what addons I have installed? If so: how and why does it work that way?
Asked by: Guest \| Views: 141

Total answers/comments: 2

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Guest [Entry]

"Yes, a website can detect that Adblock Plus is installed by referencing a Javascript file known to be blocked by Adblock Plus (because it matches a default rule) and check if this Javascript code has run, by checking a variable for instance.

See an explanation here: How To Detect Adblock Plus

And more details at Wikipedia: Adblock - Detection."

Guest [Entry]

Without using any trickery, most browsers advertise installed plugins (like Flash), but not add-ons (like Adblock, which is detectable by other means like Snark answered). See Panopticlick, and help their investigation...