I'm following this article. I am looking for my Stripe signing key, as needed here:
$ firebase functions:config:set \
keys.webhooks="your_restricted_key" \
keys.signing="your_signing_key" <--
At first I thought it was my publishing key, but after trying it I got the error in this question.
SOLUTION:
Go to:
Endpoints receiving events from your account
at dev/webhooks on stripe console.
click into your webhook:
it is under: Signing secret