您可以从AuthResult获取uid，以在Firestore中创建文档.setData更新文档，或者在这种情况下，创建一个文档，因为它不存在.确保安装cloud_firestore_package.

You can get the uid from AuthResult to create a document in Firestore. setData updates the document or in this case, creates one because it doesn't exist. Make sure to install cloud_firestore_package.

UserCredential result = await FirebaseAuth.instance.createUserWithEmailAndPassword(email: _email, password: _password);
User user = result.user
await FirebaseFirestore.instance.collection('users')
      .doc(user.uid).set({ 'firstName': _firstName'})