There are plenty of reasons Certificate Error Navigation Blocked comes up. Before you start digging around and trying to fix the problem, you should first consider that there may not actually be a problem. Maybe the site isn’t configured properly, and your browser is doing its job.
There are common causes, though, that you can fix. Many involve something being configured incorrectly on your system, like your clock. Missing updates can also raise the error. Antivirus programs and firewalls are common causes as well.
Aside from a website potentially experiencing server-side SSL issues, it’s possible your Firefox settings are causing a miscommunication between the website server and your computer. This is a far likelier case if many different websites are throwing the error code SSL_ERROR_NO_CYPHER_OVERLAP.
You should always keep Firefox up to date anyhow, but an outdated version of Firefox is a potential cause of Firefox error SSL_ERROR_NO_CYPHER_OVERLAP appearing.
If either TLS or SSL3 are disabled or otherwise misconfigured in your Firefox TLS settings, the error code will also commonly spring up. Finally, any website that uses RC4 (Rivest Cipher 4) in its encryption will run into issues with Firefox TLS no matter what. This is because RC4 was prohibited from TLS in 2015.
One easy fix for many issues is to simply reset the parameter RAM to its default state. This will cause some data to be lost, specifically the date, the time, and the startup volume selection. Luckily, you can easily correct these settings using your Mac’s System Preferences.
The steps needed to reset the parameter RAM are the same, regardless of whether your Mac uses NVRAM or PRAM.