Due to an expired certificate, many add-ons are disabled for many Mozilla Firefox users. Reactivating and reinstalling are also not options. Mozilla says it is working to resolve the problem as quickly as possible.
More information besides the fact that it concerns a certificate problem is not available at the time of writing. Mozilla keeps users informed via a status page, but it is not always easy to reach because of all the interest. There is no indication of how long the problem will last.
Firefox add-ons must have been digitally signed since version 48, which was released in 2016. This is to prevent malware spread via rogue add-ons. ZDNet writes that a temporary solution is to reset the computer clock and date to before May 4, 2:00 AM, when the problem started to occur. Workarounds are also shared which mean that the certification requirements are temporarily eliminated. This ensures that add-ons work again, but also entails security risks.