I think the reason is probably network.negotiate-auth.delegation-uris and network.negotiate-auth.trusted-uris in about:config, and your authentication is failing.
Since Chrome, Edge, and IE uses Windows certificate infrastructure and Firefox uses its own, it won't understand directive perhaps pushed through your company's group policy. You can specify address of your intranet site, in style of https://.example.com (where subdomain of example.com is covered in this case) and it should work.
As for certificate, is it using internal certificate of some sort? If that's the case, you will have to load it to Firefox's NSS.
