Firefox shows Secure Connection Failed when it cannot complete the secure certificate check for a site. Sometimes the website has a real certificate problem, and sometimes the computer, antivirus, VPN, clock, or network is interfering.
Does this affect you?
Use this for Firefox on Windows or Mac when a site fails with codes such as SSL_ERROR_RX_RECORD_TOO_LONG, SEC_ERROR_EXPIRED_CERTIFICATE, or MOZILLA_PKIX_ERROR_MITM_DETECTED.
Check the common causes first
These quick checks fix many certificate errors.
- Make sure the computer date, time, and time zone are correct.
- Hard reload the page with Ctrl+Shift+R on Windows or Cmd+Shift+R on Mac.
- Open the site in a Private Window to rule out cached data or extensions.
- Try the same site on phone mobile data. If it works there, focus on the computer or network.
Review Firefox security settings
Firefox has its own certificate behavior, so the error can appear only in Firefox.
- Open about:preferences#privacy.
- Scroll to Certificates and open View Certificates.
- Look for unfamiliar certificate authorities, while remembering antivirus and work tools may add expected entries.
- Temporarily disable HTTPS-Only Mode to test whether that setting is involved.
- Reload the site, then restore your preferred HTTPS setting.
Rule out security software, VPN, and network interference
HTTPS scanning can trigger Firefox-specific warnings.
- Temporarily disable antivirus HTTPS or SSL scanning.
- Disconnect any VPN.
- Restart the router.
- Reload the page.
- If the site works, re-enable protection and add an exception instead of leaving protection off.
More control
MITM warnings
MOZILLA_PKIX_ERROR_MITM_DETECTED usually means something between Firefox and the site is replacing the certificate, often antivirus software or a managed network inspection tool.
Do not click through blindly
Continue past a certificate warning only when you understand the cause. Do not enter passwords or payment details on a site with an unexplained warning.
Refresh Firefox
If many sites fail and nothing else helps, open about:support and use Refresh Firefox. It resets Firefox settings while keeping bookmarks and passwords.
Sources
- Mozilla Support – Secure Connection Failed error message (2025)
- Mozilla Support – What do the security warning codes mean? (2025)
- Mozilla Support – Troubleshoot Firefox issues caused by malware (2025)