We're running SharePoint 2010 and when I try to go to our intranet site, it asks for my Windows credentials. I've looked around a little and have seen some changes that need to be made in Firefox but that hasn't fixed it. Before upgrading to 2010 a few years ago, I had the same issue but making those changes in Firefox resolved it. The settings in Firefox I've changed is adding our companyweb URL into the trusted URIS settings.

HI,I would recommend to add the site in the trusted zone of Firefox and IE also.Apart from this disable the loop back check to stop asking the password.Please check the link below that explains more in details about the issue.

http://expertsharepoint.blogspot.my/2013/10/sharepoint-site-is-prompting.html