Hi, welcome to Firefox! You can look for add-ons on [https://addons.mozilla.org/firefox/ addons.mozilla.org] (or just select Extensions and themes in the Firefox menu and use the search bar on the page that appears). To find an ad blocker, you can literally use [https://addons.mozilla.org/firefox/search/?q=ad%20blocker "ad blocker"] as a search query. Top recommended ad blockers are [https://addons.mozilla.org/firefox/addon/ublock-origin/ uBlock Origin], [https://addons.mozilla.org/firefox/addon/adguard-adblocker/ AdGuard] and [https://addons.mozilla.org/firefox/addon/ghostery/ Ghostery], but there are also lots of others to choose from!