One of those things which you need to do every so often and it takes longer then it should to remember how... and even Google does a poor job on a quick search of returning the simple step you need.

After you type in the FTP://site/dir to your browser (and probably gotten a credentials required message) you need to go to the file menu.  On the File menu select "Login As..." and then enter the credentials for the FTP site you are trying to access.  It's simple just not something most of us need more then about once or twice a year.