Try one of these articles:
- How to troubleshoot the error code "SEC_ERROR_UNKNOWN_ISSUER" on secure websites This article explains why you might see the error code "SEC_ERROR_UNKNOWN_ISSUER" on HTTPS websites and how you can troubleshoot it.
- Fix problems connecting to websites after updating Firefox Learn to troubleshoot and fix problems connecting to websites that start immediately after updating to a new version of Firefox.
- Configure firewalls so that Firefox can access the Internet A firewall controls what programs can access the Internet. Find information about configuring your firewall to allow Firefox access to the Internet.
- Unable to access secure (HTTPS) sites in Firefox 43 This article explains how to fix the error SEC_ERROR_CERT_SIGNATURE_ALGORITHM_DISABLED so you can access secure (HTTPS) sites in Firefox 43.