Firefox has released version 66.0.4 that fixes the issue. If you are not using the version released by Firefox but a version like Firefox ESR or a custom version from your Linux distribution. You can still fix the issue by manually importing the new valid security certificate into Firefox.
The new security certificate and instructions on how to do so is located at:
1. Go to the above link and make sure you understand what you are are about to do.
2. Download and save the following file:
3. From within Firefox go to Preferences -> Privacy & Security -> Certificates – > View Certificates and go in the Authorities tab and click on Import.
4. Locate the file you downloaded and select it.
5. Restart Firefox and you should be able to re-enable your add-ons and themes
6. When your software vendor releases their version of Firefox that solves this issue. You should go back and remove the certificate before installing the new version of Firefox. You don’t want to cause any issues when Firefox tries to add the certificate but it already exists.
The certificate located in the above file use the following identifiers.
Common Name: signingca1.addons.mozilla.org
Organization: Mozilla Corporation
Organization Unit: Mozilla AMO Production Signing Service
Valid From: April 3, 2015
Valid To: April 3, 2025
Issuer: root-ca-production-amo, Mozilla Corporation
Serial Number: 1048584 (0x100008)