Try one of these articles:
- Connectivity or crashing issues after Firefox 43 update After the update to Firefox 43, users with ''Nvidia Forceware Network Access Manager'' might observe connectivity issues and crashing. See here how to fix.
- 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.
- 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.