This is because Firefox does not support NTLM very well, also known as Windows Authentication. If you are looking for firefox ntlm , you will find many resources that will provide you with options on how this might work.
I change jobs every six months or so and never could get it to work reliably. Sometimes it is impeccable, sometimes I always get a hint, no matter what. However, they are gradually improving support with new versions of Firefox, so you can try to completely remove and reinstall Firefox.
source share