Thanks for reaching out to us! We are aware that most add-ons on Firefox including Adblock Plus were recently deactivated due to an issue with the browser’s signing certificate expiring. Mozilla has released an official update to deal with this issue https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/. If the update has not yet reached you, please try updating manually using this link https://www.mozilla.org/en-US/firefox/new/. Let us know if this resolves your issue! Reach us via firstname.lastname@example.org if you still having this issue.