Fix NS_ERROR_NET_INADEQUATE_SECURITY in Firefox Quickly

1. Disabled TLS 1.0 or 1.1 — The Real Culprit

Firefox 74 and later disabled TLS 1.0 and 1.1 by default. If the server you're hitting only speaks those old protocols, Firefox flat-out refuses to connect. You'll see NS_ERROR_NET_INADEQUATE_SECURITY almost every time this happens. Some intranet sites, legacy web apps, and older routers' admin panels still use TLS 1.0. The server you're trying to reach might be one of them.

To force Firefox to accept TLS 1.0 or 1.1 temporarily:

1. Type about:config in the address bar and press Enter. Click through the warning if it shows up.
2. Search for security.tls.version.min (this sets the minimum TLS version).
3. Double-click it. Default is 3 (TLS 1.2). Change it to 1 (allows TLS 1.0).
4. Search for security.tls.version.enable-deprecated and set it to true.

Now reload the page. If it works, the server's TLS config is the issue. Don't leave those settings changed permanently — it weakens your browser security. Revert them after you're done.

Pro tip: If you see this error on every site, especially big ones like Google or Facebook, the server isn't the problem. Skip to the next fix.

2. TLS Cipher Suite Mismatch

Even if the server supports TLS 1.2 or 1.3, it might use a cipher suite Firefox no longer trusts. Firefox dropped support for several older ciphers after version 60. The most common one that breaks sites is TLS_RSA_WITH_AES_128_CBC_SHA. Servers that don't support modern ECDHE or GCM ciphers will trigger this error.

You can check the server's cipher list using SSL Labs. If the server only offers CBC ciphers, you're stuck — the site admin needs to update. But for your own testing, you can temporarily re-enable old ciphers:

1. Go back to about:config.
2. Search for security.ssl3.ecdhe_rsa_aes_128_sha — this is the old CBC cipher. Set it to true.
3. Also set security.ssl3.ecdhe_ecdsa_aes_128_sha to true if needed.

If the site loads, the server's cipher config is outdated. Again, revert after troubleshooting.

3. Proxy, VPN, or Antivirus Interference

I've seen this a lot. A proxy or VPN that's doing SSL inspection (like corporate firewalls, some VPN apps, or antivirus suites) can inject a custom certificate that Firefox doesn't trust. The result? Firefox sees a mismatched certificate chain and throws NS_ERROR_NET_INADEQUATE_SECURITY. Antivirus products like McAfee WebAdvisor, Norton, or Avast's HTTPS scanning are common offenders.

Try this in order:

• Disable your antivirus's HTTPS scanning — look for settings like "SSL scanning", "HTTPS filtering", or "web protection". Turn it off temporarily.
• Check Firefox proxy settings — go to Settings > Network Settings. Set it to "No proxy" if you don't use one.
• Temporarily disconnect your VPN — if the site loads after disconnecting, the VPN's SSL inspection or routing is the issue.
• Flush DNS — open a terminal and run ipconfig /flushdns on Windows, sudo dscacheutil -flushcache on macOS, or sudo systemd-resolve --flush-caches on Linux.

If none of that works, test with Firefox's safe mode (Help > Restart with Add-ons Disabled). If the error disappears, an extension is messing with your connection. Disable them one by one.

Quick-Reference Summary Table

If you've made it through all three and the error still shows up, you're looking at a deeper network issue — check your company's firewall or ISP for HTTPS filtering. But 9 out of 10 times, it's one of these.