For security purposes, you will need to verify that you own the domain where your application is hosted.
There are three options for verification:
Add a TXT record in your domain control panel
Upload an HTML file to your root domain
Add a meta tag to your root HTML page
You'll be prompted to verify your domain when you first sign up, or alternatively you can do this via the domain settings.
If you need to change your domain, you can verify a new domain in the domain settings.
Firelab doesn't currently support testing a localhost address, as your app will need to be publicly available. If you don't want to test your production app, then it's recommended that you test your staging app via a domain.