Try one of these articles:
- Refresh Firefox - reset add-ons and settings Restore Firefox to its default settings while saving essential data. This can fix slowness, crashing, search hijacking and other problems.
- 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.
- Firefox Health Report - understand your browser performance Firefox Health Report lets you know how well your browser is performing and what steps you can take to improve it.
- What do the SSLv3 error messages mean on Firefox? When accessing a site with SSLv3, Firefox will block it and display an "Unable to connect securely" message.