What site is this? Firefox does have simple adblocker built in but for most part it doesn't affective internet usage. Did you check there is no other Addon that is causing this as well and if the A/V isn't interfering with internet usage.

Often sites are detecting that one of their scripts was blocked, and guesses that you have an ad blocker. But there are a variety of add-ons that can block scripts, and even Firefox's Tracking Protection feature.

Could you check:

(A) The shield icon toward the left end of the address bar usually turns a bit purplish when Tracking Protection is blocking content. Click the icon to learn more or make an exception. See: Enhanced Tracking Protection in Firefox for desktop.

(B) Extensions such as Adblock Plus, Blur, Disconnect, DuckDuckGo Privacy Essentials, Ghostery, NoScript, Privacy Badger, uBlock Origin or uMatrix may block required elements on a site. Usually you can find a toolbar button for each add-on to manage blocked content in a page. There may or may not be a number on the icon indicating the number of blocked items; you sometimes need to click the button to see what's going on and test whether you need to make an exception for this site.

Any difference so far?