Secure Connection Failed
Firefox couldn't set up an encrypted connection to the site, so it stopped. The detailed code says whether it's a certificate, protocol or network problem.
Quick checks
- Click Advanced or More information to see the error code.
- Check your date and time.
- Try another network.
Fixes on Mac
SEC_ERROR_UNKNOWN_ISSUER from antivirus or proxies
- Firefox keeps its own certificate list. If security software or a corporate proxy inspects HTTPS, Firefox may not trust its root.
- In
about:config, setsecurity.enterprise_roots.enabledtotrueso Firefox trusts certificates installed in the macOS keychain.
Clear Firefox's data for the site
- Firefox › Settings › Privacy & Security › Cookies and Site Data › Manage Data, remove the site.
Check proxy settings
- Firefox › Settings › General › Network Settings › Settings: No proxy, or system proxy settings.
Fixes on Windows
Same steps
- Check antivirus HTTPS scanning and Firefox's enterprise roots setting.
Other devices
Phones
- Update Firefox and check the date and time.
Still not working?
If SEC_ERROR_UNKNOWN_ISSUER appears for one site in every browser, the site isn't sending its intermediate certificates. Site owners should serve the full chain.
See what’s happening
LightAtom's connections view shows whether your browser connects to a proxy address instead of the site's servers, and which process runs that proxy.
Frequently asked questions
What does SEC_ERROR_UNKNOWN_ISSUER mean?
Firefox doesn't recognise the authority that signed the certificate: a missing intermediate on the site, a self-signed certificate, or interception software Firefox doesn't trust.
Why does Firefox fail when Chrome works?
Firefox uses its own certificate store by default, while Chrome on a Mac uses the system keychain. Certificates installed by security software or IT are only in the keychain.
Is it safe to add an exception?
Only for devices you own on your own network. Never for public sites where you sign in or pay.