The perfect solution would be if Airtable implemented webhooks. This way your Firebase Cloud Function would be invoked any time an update is done.
The solution right now would be a combination of cron job and a hash.
You could create a field with a formula that summed the data of all the other fields and returned a number.
In another table you could make a rollup of that field and sum all the values. This way, whenever a new record is added or a field is updated the value of the rollup would change.
Now, you could create a cron job that would ping your cloud function once an hour for example.
Then, your cloud function would check what happened.
Not a great solution,