Mozilla might have a major issue in their hands as the company forgot to renew their security certificate. The company was supposed to renew a certificate that is used to sign Firefox add-ons but has forgotten to do so.
According to an announcement on Mozilla Discourse (which seems to be having some issues as well), the certificate expired causing new add-ons to fail and is stopping the existing add-ons from working. The issue was first reported on Reddit (via Neowin) by Firefox users who claim that add-ons such as content blockers, password managers, and download managers have stopped working completely.
Mozilla is currently working on fixing the issue and in the meantime, you can head to r/Firefox for some temporary fixes to the issue.