AFib Firebase Firestore is AFib state library which extends your project-specific AFib command with an affs:generate subcommand. That subcommand generates AFib queries which interact with Firebase's Firestore database.
If you are not familiar with AFib, you should start by reviewing the YouTube trailer and YouTube tutorial found on www.afibframework.io.
AFib Firebase Firestore itself has a brief written guide, and as it states, you are best off starting with one of AFib's standard signin-related project styles, either app-starter-signin or app-starter-signin-firebase.
Creating a new AFib project is covered in AFib's overall written guide..