Download Older Unity Firebase SDKs

The official firebase website has a link to download the latest version of their SDK for Unity. The page is here and links to the following URL.

Download Latest Unity SDK
https://firebase.google.com/download/unity

Older versions

To download older versions, you can use a direct link like below.

Download arbitrary Unity SDK
https://dl.google.com/firebase/sdk/unity/firebase_unity_sdk_X.YY.Z.zip

For example to download firebase SDK version 6.15.2, the direct link would be:

https://dl.google.com/firebase/sdk/unity/firebase_unity_sdk_6.15.2.zip

This information came from this Github issue.