Are they being that specific, or just saying "an ad blocker"? Many sites, if they can't load all their ads and trackers, will accuse you of using an ad blocker. The culprit might be a different add-on or even Firefox's own Tracking Protection feature. Other extensions to look for are DuckDuckGo Privacy Essentials, Ghostery, Privacy Badger, uBlock Origin, Blur, and Disconnect. If you don't have any of those, either, hmm. Here's how you can check whether it is caused by Tracking Protection: The shield icon toward the left end of the address bar (next to the lock) usually turns a bit purplish when content is blocked on the current page. Click the icon to learn more or make an exception. See: [[Enhanced Tracking Protection in Firefox for desktop]].