Mozilla Forgot To Renew Certificate, Auto-disabled All Firefox Addons until May 4, 2019. The browser assumed the add-ons installed by the user on their browsers to be from a 3rd party. Mozilla started using a digital certificate in order to enforce the use of official extensions from https://addons.org/en-US/firefox/extensions/. All extensions hosted on the site were deemed by Firefox 66.0.3 and Firefox 60.06.1 to be invalid, hence installation was disabled since May 4.”]
Source: https://hackercombat.com/mozilla-forgot-to-renew-certificate-auto-disabled-all-firefox-addons/