We will create a FastAPI app with the following core classes, functions, and methods:

1. User: A dataclass representing a user with attributes like id, email, password, and name.
2. create_user: A function to create a new user in Firebase.
3. get_user: A function to get a user from Firebase by their email.
4. authenticate_user: A function to authenticate a user using their email and password.
5. request_openai: A function to send a request to OpenAI's ChatGPT3.5 LLM with the given API body and return the simplified text.

Now, let's create the necessary files and their content.

main.py