TFS uses a version of NTLM over unsecure HTTP and Firefox doesn't want to allow this by default. Of course, Firefox doesn't tell you it's forbidding something; instead it just acts like you got a 403. Here's how to fix it so you can use FF on TFS: In your address bar, go to "about:config" You should get a little search bar and a giant list of properties. In the search bar, type "ntlm" Look for the property "network.negotiate-auth.all... and double-click the "false" in the Value column. ......