Around a week and a half ago on Saturday May 4, a certificate used to sign Firefox add-ons expired and subsequently blocked new add-ons from being installed and existing add-ons to stop working altogether. While there were several workarounds that could be applied before Mozilla was able to issue a hotfix release in the early hours of Monday morning, which was followed up by another a couple of days later, this left those on older versions to search for a temporary workaround.
Now though, and after pledging to release fixes for older and unsupported versions of Firefox, Mozilla has released add-ons that fix the certificate signing on Firefox 47 - 56, one for those on version 57 - 60, and lastly for versions 61 to 65. For whatever reason those people are on those older versions, they are now not forced to apply the temporary workaround or upgrade to the latest version 66.0.5.
The extension installs a new certificate and re-enables extensions and themes that were disabled. The add-ons can also be installed in Waterfox versions that are affected by the issue as well to patch the issue in the third-party browser.
You can grab the add-on for your version with the links below.
Source: Disabled Add-on Fix for Firefox 47 - 56 | 57 - 60 | 61 - 65 via gHacks