Thank you for reporting this! It appears that Firefox recently experienced an issue related to extensions. This affected all extensions, not just AdBlock. A fix has been rolled out with the latest release of Firefox, so as long as you're using the latest version, you should be all set now. Here's some more information about what happened: https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/

-Christina, Director of Support @ AdBlock