To add funds to your Account from the Control Pane
Login to your Customer Control Panel. See details
Point to My Billing and then click Add Funds.
There you would be presented with various methods available to Add Funds to your Account with Firebase. You can choose one of the options and Add Funds to your Account.
Adding Funds to your account does not automatically settle any Invoice/Debit Note pending against your Account. You will have to manually pay for it. See details