[MiguelHudnandez]

An address can receive multiple payments, but the most likely use case from that is mining output, or recurring payments from the same sender.

A wallet owner is able to generate new addresses at any time, and generally does so in order to verify that a payment is coming in for a specific transaction. For example, a Bitcoin ATM operator wouldn't generate a payment address until a patron is standing at the kiosk. As soon as a transaction is verified, the machine would dispense whatever amount of money and future payments to that address would probably be ignored.

You are right that Mozilla probably isn't even operating a bitcoin wallet in this case. They are probably just getting USD from Coinbase.