Open source SaaS – how to handle Firebase auth credentials, so people can develop backend (.NET), frontend (React/NextJS) or mobile (Flutter)?

I am building the non-profit SaaS, which we will be hosting with our organisation in the cloud, but we want to allow people to contribute, so I need to somehow allow them to run the components locally for development and testing.

I came across this issue of Firebase configuration, which is required for auth to work.

Can you recommend any good way of handling that?
I need to cover the following scenarios:

• someone wants to work on the backend
• someone wants to develop a frontend part (in this case, they probably need to change config on local backend but also on their local frontend)
• someone wants to develop a mobile app (Flutter) – similar issue as above – they would need to change credentials on the backend and in the mobile app

I believe there can be similar issues daily at the companies, but inside the company, as there is some trust established, many credentials can be shared, while with publicly open code and development, any credentials cannot be exposed as they would be immediately abused.