Managing permission to edit user profiles & The downsides of not separating frontend from backend

Hi there,

I am using React to build a website and have the following questions:

1. How can I only allow the owner of a profile to edit it and make it read-only for other users (e.g. LinkedIn profile)?

2. What are the downsides of directly calling firebase's methods in my frontend to (1) store data into my firebase database and (2) retrieve and display data from the database without using a backend?

Thank you so much for bearing with the beginner questions.