If it's a one-time thing you can probably just run a curl command.
I built Bracket to do this exact thing. You just have to input your Airtable API key and connect it to your Firebase. It handles syncs in either direction too. So sending data to Firebase in JSON format or sending data in JSON from Firebase to a relational format in Airtable is possible
There's also features that you can customize. If you only want to sync certain fields in two directions and make other fields read-only in Airtable, or if you want to write queries to make the data you pull into Airtable very customizable, then that's all possible!