Ad blockers are Extensions so that's where they'll be listed. Type about:addons into the Search/URL bar and you'll see a list of any you have installed. There are three horizontal dots to the right of them and clicking those gives the option to remove that extension.

Note that Enhanced Tracking Protection also can make a website think that you use an ad-blocker.

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/total-cookie-protection-and-website-breakage-faq
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track