If this error occurs when you are on a Corporate VPN, where browser traffic is captured by a MITM (Man-In-The-Middle), you may need to change any or all of these settings in the Firefox about:config page,
security.tls.hello_downgrade_check = false (this should definitely fix it)
security.tls.version.max = 3 (this should definitely fix it)
security.osclientcerts.autoload = true (this might fix it if it is OS cert-related)
The reason this may occur is because the security device is incapable of providing valid TLS 1.3 HELLO messaging.
Thank you! This fixed a problem that our IT dept could not even figure out. Could not get into Google after updating firefox. This worked great. Thanks alot.
Thank you so much for letting me know. Glad that it fixed it! 🙂