You won't have direct access to the Firebase users table (node) so that won't serve as a trigger.
However, traditionally, Firebase apps have a /users/ node where other data about a user would be stored; first and last name, address, love of pizza etc. And that node would have a key that matches their uid.
That's the node you want to use to trigger Zapier; when you create a new firebase user, you also add their data to your /users/ node to then trigger the email.
One thing to note is that Zapier will/can automatically remove a trigger node in Firebase so you may just want to dump the email in a /need_to_send_email/ node with the email address and let Zapier trigger and then remove that data as it would no longer be needed.
typical Zapier structure within firebase
app_name
zapier_queue
out
email: "somedude@cooldudes.com"