Hat tip to this blog post. You have to add the URL of you ADFS server to the
network.automatic-ntlm-auth.trusted-uris setting in
about:config. Official documentation from Mozilla is here. Firefox, by default, does not negotiation authentication with a web server nor does it send NTLM responses. You have to explicitly whitelist sites you want to do this with.
Bear in mind you can’t do a domain wildcard either. So no “*.raxnet.global”, it has to be either “adfs.raxnet.global” or “https://adfs.raxnet.global”. Not like IE in that respect.
If in an enterprise of Windows computers you can manage this via GPOs. I don’t know how I missed it, but Firefox supports Group Policies since March 2018. Download the templates here. And while you are at it, you can also get it to pull in the enterprise root certs. Neat!
Also, I learnt the hard way that the settings for whitelisting sites don’t seem to take effect in private mode. So don’t waste time making these changes in a GPO and testing in private mode. :) if at all you need to re-test, you have to clear the history.