Unity and Firebase - Package Manager Resolver, uninstalling the following packages: Firebase Authentication - firebase

Question

I'm using Unity 2019.3.4f1 and Firebase Package 6.15.2 and when I import the Custom Package the Package Manager Resolver asks to change the "/manifest.json".
When I click "Add Selected Registries", Unity starts to Uninstall the packages and stay on its hours with this message: "Uninstalling the following packages: Firebase Authentication"
Package Manager Resolver
I can't deploy or play the game without this message appears.

Answer 1

What you're running into is that newer versions of Unity support a package manager that makes it easier to install and manage dependencies. Newer versions of the Firebase SDK can optionally take advantage of this.
I can't quite tell what your specific issue is, but there are a couple routes forward.
The simplest is to just click "Disable Registry Edition." If you don't feel like messing with this system at all, just click this button and work with Unity packages as you're probably expecting.
Otherwise, you can click "Add Selected Registries." This will kick off a somewhat complex process where:
The Firebase plugin adds a "Scoped Registry" to Unity (this is that code you see in the "After" pane). This tells Unity about Google's package repository.
The Plugin then looks for any package that is included in the registry and uninstalls it to avoid having it included twice.
The Plugin adds the package it uninstalled to your package manifest. This lets you maintain and update the Firebase plugin right in Unity with the "Package Manager" window:
There are plenty of reasons why you'd want to do this. First, the Firebase Unity SDK is HUGE on disk -- much larger than the SDK is even in your project. The reason is that it has redundant copies of every Firebase library for both legacy .NET3 projects and for modern .NET4 projects. Each unitypackage also has to include all of its dependencies -- that means that FirebaseCore is redundantly included in every unitypackage.
Second, what gets me the most, is that the Firebase plugin is too big to fit into a GitHub repo without Git LFS. This is because one or more of the libraries needed to support Linux is larger than a single source file can be. When you use the package manager, this stuff is kept out of your source repository (if you don't commit the Library/ directory, which you shouldn't) keeping your size in the cloud down and making this workaround unnecessary.
Third, as illustrated in the screenshot I included above, it's just easier to upgrade and downgrade the Firebase SDK as needed when you use the package manager. You no longer have to try to remember which Firebase libraries you've installed, you can see them in a neat list! You can also easily uninstall Firebase features that you don't need without worrying too much about large dependencies laying around (you still have to manually clean up some native plugins).
You can also perform all of this manually! Instead of downloading the Unity SDK, you can manually perform the steps as outlined here. Namely you can add:
"scopedRegistries": [
{
"name": "Game Package Registry by Google",
"url": "https://unityregistry-pa.googleapis.com",
"scopes": [
"com.google"
]
}
]
to the end of your Packages/manifest.json as indicated in that popup window. Then install and manage the Firebase plugin that way without worrying about migration at all.
EDIT:
I also should mention that if you do think that you're running into a bug, the system responsible for the dialog you're seeing is known as the "External Dependency Manager for Unity." You can file issues directly on its GitHub page.