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.
- 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.
- Websites don't load - troubleshoot and fix error messages Are you having trouble getting a website to load? This article will show you how to fix the problem so you can get back to surfing the web.
- 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.