Hello everyone, It is my first time here. I am asking for your help on something that has been bugging me for a week: I have recently deployed Firefox ESR 78.0.2 in my company after spending months studying about configuration files, policies file, UEV etc. and it works ! My problem now is about SSO with ADFS 3.0: no matter what I try, I either get a blank page or a Forms Based Authentication prompt when accessing a site that is configured for adfs sso and works seamlessly with IE 11 and Chrome. What I want to achieve: SSO authentication using Kerberos (not NTLM) against ADFS '''without''' setting the ''ExtendedProtectionTokenCheck'' parameter to "None". After countless research on the Internet, here's what I tried: - add "Mozilla5/0" "Firefox" and "Firefox/78.0" to the adfs ''WIASupportedUserAgents'' (and restart ADFS service of course) -> makes chrome sso work, but not Firefox - mess with those preferences: ''network.negotiate-auth.trusted-uris / network.negotiate-auth.delegation-uris / network.negotiate-auth.allow-proxies / network.negotiate-auth.allow-non-fqdn / network.negotiate-auth.using-native-gsslib / network.auth.use-sspi / network.automatic-ntlm-auth.trusted-uris / network.automatic-ntlm-auth.allow-proxies / network.automatic-ntlm-auth.allow-non-fqdn / network.auth.force-generic-ntlm / signon.autologin.proxy'' - changing my user agent by setting preference ''general.useragent.override'' to "Firefox" - allow every cookies possible.. - troubleshoot http requests / response with ''SAML Tracer extensions for Firefox'' When I get a blank page (typically when ''network.auth.force-generic-ntlm'' is at ''false'', which is what I want), I get an error 500 (see screenshot) When I get a Forms Based Authentication prompt, I get an error 401 Unauthorized (which I think is normal since FBA is not set up in ADFS parameters). In both case I can see that Firefox is atleast trying to negociate authentication first with Kerberos, then with NTLM. I am frustrated because I see many posts where people resolved their issues only messing with the ADFS WIASupportedUserAgents parameter and the FF prefs network.negotiate-auth.trusted-uris / network.negotiate-auth.delegation-uris Of course, if I disable the ADFS "ExtendedProtectionTokenCheck" for testing, everything works. Does anyone know if there is something else that can interfere with Firefox's SSO ? Could it be another FF preference ? Or maybe my ADFS is misconfigured for what I want ? Best regards