AFAIK the webinterface has no username, only a password.
When logging in to fireplay (which has the same password) I only enter the password and I get access.
The standard password is usually ‘mt-daapd’ or ‘firefly’. But as you already have tried, you should look in your config file.
The config file is probably located at /etc/mt-daapd.conf
Hope this helps. 🙂