How do I fix the problem behind "THIRD_PARTY_AUTH_ERROR" when using Google's API Explorer to send a Firebase Cloud Messaging message? - firebase-cloud-messaging

Question

I am trying to create a very simple Web app that can send and receive messages (including Push notifications) via Firebase Cloud Messaging. I want to try testing the ability to receive test messages sent from the Google API Explorer. However, I am running into a curious error as just about any API call from the relevant API URL will return the following error:
{
"error": {
"code": 401,
"message": "Auth error from APNS or Web Push Service",
"status": "UNAUTHENTICATED",
"details": [
{
"#type": "type.googleapis.com/google.firebase.fcm.v1.FcmError",
"errorCode": "THIRD_PARTY_AUTH_ERROR"
}
]
}
}
For reference, here is the message that I am trying to send:
{
"message": {
"token": "[FCM_TOKEN]",
"notification": {
"title": "New Notification",
"body": "Sample Text"
}
}
}
I also note that when I add "validate_only": true to the message, that the error goes away and the send request succeeds (though this is of no help, as that disables actually sending the message).
I am stumped, becuse the hint that pops up suggests that my OAuth2 credentials are invalid, though I leave the OAuth2 credential option checked and go through the authorization popup to provide my credentials.

Answer 1

Are you running the web app on Firefox, perhaps?
I have been running around my tail with this same issue for days now, and today I've found that there is an issue on Firefox, while this does work on Chrome:
Similar stack overflow question
The issue on the Firebase repository on Github
Although not a proper answer, I hope this helps.