The error code like SEC_ERROR_UNKNOWN_ISSUER might appear on your screen while you are using Firefox as your browser and make you disabled to browse websites like Google, Facebook or YouTube. You need to check your extension as well as your connection settings to resolve the matter. You can view and remove unwanted extensions in your add ons page and see whether any improvement is done or not. You should also check your LAN setting to confirm whether it is working properly or not. If it is working fine and your issue still exists, you better contact our tech support, team. They will detect the exact reason and provide you the right solution at the earliest possible time.