[HOW] Find Old Firebase SDK Versions for Unity3D

The SDK link on the Getting Started with Firebase page automatically downloads the latest Firebase SDK. Quite annoying if you're looking for an older version if you want to re-import an older SDK for your project for example. Luckily there's a way to download the older versions. Just enter the version number that you want in this link  https://dl.google.com/firebase/sdk/unity/firebase_unity_sdk_{Version number}.zip.

For example, if you're looking for version 6.0.0, the download link would be https://dl.google.com/firebase/sdk/unity/firebase_unity_sdk_6.0.0.zip. That's it!

Source: https://stackoverflow.com/questions/49921164/old-versions-of-firebase-for-unity-firebase-sdk-4-5-0