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.allow-insecure-ntlm-v1" and double-click the "false" in the Value column. It should boldface and turn to "true". The boldface indicates a user-set property.
- Restart FF and you're good to go.