Mozilla Firefox users are discovering that all of their addons were suddenly disabled. Mozilla has issued a temporary fix for this issue. An intermediate signing certificate used to sign Mozilla addons expired on 5/4/19 at midnight UTC. Mozilla says they are looking into the issue and are issuing temporary fix through their Studies system. Users can download addons directly from the Firefox Add-on site by right-clicking on the Add to Firefox button and select Save Link As as shown below.
Source: https://www.bleepingcomputer.com/news/software/firefox-addons-being-disabled-due-to-an-expired-certificate/