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
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.
Don’t want to have to import your root certificates in Firefox in addition to IE? With this setting you can just set Firefox to use Windows’ native certificate store. I seem to need to do this frequently, with all of my reinstalls of Firefox.
Go to Firefox, type
about:config in the address bar and hit Enter, agree to the disclaimer, then search for
security.enterprise_roots.enabled and set it to True.
More information here: Mozilla Wiki: Add Root to Firefox
To configure it for NTLM (used in enterprise for authentication on internal networks), this guide has more information (How to Enable Automatic NTLM Auth. First set
network.automatic-ntlm-auth.allow-non-fqdn to True, then go to
network.automatic-ntlm-auth.trusted-uris and add your domain, such as, “intranet.local, intranet”.