That sounds correct.
The fact that the error is being triggered from the ‘home’ form rather than the server code, though, makes me think that the error is unrelated to the secret/.json stuff. Did you do the following part of #3 in my instructions?
copy the firebaseConfig info into the
init_firebase
method of the ‘home’ Form in your Anvil app
In any case, I suspect the issue is with something on your client-side setup rather than the server-side.