This addon, which I've been using for years, is forcibly disabled in FF 66.0.3. Uninstalling it and reinstalling does not work either. When I click "+ Add to Firefox", i get an error "Download failed. Please check your connection."

edit: This whole issue is unintended. Mozilla is aware of it and is working on a fix right now. Don't uninstall the addons, they will work again when the issue is fixed.
Mozilla thread: https://discourse.mozilla.org/t/certificate-issue-causing-add-ons-to-be-disabled-or-fail-to-install/39047
Reddit, temporary workaround: https://www.reddit.com/r/firefox/comments/bkhtv8/heres_whats_going_on_with_your_addons_being/