What exactly mean “No Outbound Networking” in free tier on Cloud Functions for Firebase?

firebaser here

Good catch, that piece of documentation probably should be clarified.

Inbound requests are allowed, as long as you're within your allowed quota. So your app can call a Cloud Function.

Outbound requests for projects on the free plan are only allowed to Google-owned services. So your Cloud Function code can only call google owned web APIs, unless it's on a paid plan.

Update: the above applies up to Node.js runtimes up to v8. From Node.js 10 and upwards your Firebase project needs to be on a paid plan to be able to use Cloud Functions. For full details on this, see the Firebase FAQ on Cloud Functions pricing.