MOZILLA_PKIX_ERROR_MITM_DETECTED — Firefox

Enterno.io Editorial Team

MOZILLA_PKIX_ERROR_MITM_DETECTED in Firefox

By Anatoly Oshmanovsky · Updated Apr 17, 2026

Key idea:

MOZILLA_PKIX_ERROR_MITM_DETECTED — Firefox saw a suspicious certificate on a domain with pinned keys (google.com, facebook.com, etc). Not a baseline SSL warning — Firefox thinks you're being MITMed. Causes: corporate proxy (Zscaler, Kaspersky AV), legit parental controls, real attack (rare). DO NOT ignore without cause.

This error blocks HTTPS access. Below: causes, fixes, working config, FAQ.

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Common Causes

Corporate proxy with TLS inspection (Zscaler, Forcepoint, Symantec)
Kaspersky Internet Security / Bitdefender with SSL inspection
Router-level parental controls
Actual MITM attack (public Wi-Fi, compromised DNS)
Hand-rolled MITM test proxy (mitmproxy, Burp)

Step-by-Step Fix

If in a corporate network — expected, contact IT
Temporarily disable antivirus SSL inspection — retest
Switch network (mobile hotspot) — if it's gone = your network
Firefox: about:preferences#privacy → Certificates → "Ask every time"
DO NOT click "Advanced → Accept risk" unless you understand the cause

Check SSL Certificate →

Related SSL Errors

CertificateExpiry, issuer, domains (SAN)

ChainIntermediate and root CA validation

TLS ProtocolTLS version and cipher suite

VulnerabilitiesHeartbleed, POODLE, weak ciphers

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TLS 1.3

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How it works

Enter domain

TLS chain verified

Expiry date & vulnerabilities

What Does the SSL Check Cover?

SSL/TLS is the encryption protocol that protects data between the browser and server. Our tool analyzes the certificate, chain of trust, TLS version, and knownvulnerabilities.

Certificate Details

Issuer, validity period, signature algorithm, covered domains (SAN), and validation type (DV/OV/EV).

Chain of Trust

Full chain verification: from leaf certificate through intermediates to root CA.

TLS Analysis

Protocol version (TLS 1.2/1.3), cipher suites, Perfect Forward Secrecy (PFS) support.

Expiry Alerts

Set up a monitor — get Telegram and email alerts 30/14/7 days before expiration.

DV vs OV vs EV Certificates

DV (Domain Validation)

Confirms domain ownership only
Issued in minutes automatically
Free via Let's Encrypt
Suitable for most websites
Most common certificate type

OV / EV

Organization (OV) or Extended Validation (EV)
Issued in 1-5 business days
Costs $50 to $500/year
For finance, e-commerce, government sites
Increases user trust

Who uses this

DevOps

SSL certificate monitoring

Security

TLS config audit

SEO

HTTPS as ranking factor

E-commerce

customer trust

Common Mistakes

❌

Expired certificateBrowsers block sites with expired SSL. Set up auto-renewal or monitoring.

❌

Incomplete certificate chainWithout intermediate CA, some browsers and bots cannot verify the certificate.

❌

Mixed content on HTTPS siteHTTP resources on an HTTPS page — the browser lock icon disappears, reducing trust.

❌

Using TLS 1.0/1.1Legacy TLS versions have known vulnerabilities. Use TLS 1.2+ or 1.3.

❌

Domain mismatch in certificateThe certificate must cover all site domains, including www and subdomains.

Best Practices

✓

Set up auto-renewalLet's Encrypt + certbot with cron — certificate renews automatically every 60-90 days.

✓

Enable HSTSStrict-Transport-Security header forces browsers to always use HTTPS.

✓

Use TLS 1.3TLS 1.3 is faster (1-RTT handshake) and safer — legacy ciphers removed.

✓

Monitor expiration datesCreate a monitor on Enterno.io — get notified well before expiration.

✓

Verify chain after renewalAfter certificate renewal, confirm that intermediate certificates are installed.

Get more with a free account

SSL certificate monitoring, check history and alerts 30 days before expiry.

Learn more

How-to

Glossary

Research

TLS Cipher Suites 2026: Distribution Report

Errors

Alternatives

Frequently Asked Questions

Why is Firefox this strict?

Mozilla pins certain root CAs and detects chain tampering. MITM on google.com is a real phishing vector.

Does Chrome show the same?

Chrome shows ERR_CERT_SYMANTEC_LEGACY or NET::ERR_CERT_AUTHORITY_INVALID marked "Pinning". Softer message.

Is a corporate proxy safe?

Depends. IT decrypts your traffic. You must trust your company's security policies and local law.

How do I know if the cert is real?

<a href="/en/ssl">Enterno SSL checker</a> from a clean network (not your office) — shows real CA.