I did the new Visual Studio 2012/13's options to select which browser you want to debug in...but I ran into a snag lately.
When Firing up a website in Visual Studio 2012 using Firefox a few weird things happened. First, I had to login...second I got an NT/Anonymous login error...all bad.
So after a few minutes of searching I found the fix. Left here for reference and hopefully it might help you. The underlying problem is actually with how Firefox handles SQL authentication. The fix is opening up about:config in Firefox and changing a few entries. Use the instructions in the link below and add "localhost".