Current state (04/2020):
- Google Cloud Functions are available in europe-west3 (Frankfurt) with Tier2-pricing but is available for Tier1 in europe-west1 (Belgium) and europe-west2 (London)
- Google Firebase Firestore can be used in europe-west2 (London), europe-west3 (Frankfurt) and europe-west6 (Zürich)
- Google Cloud Firestore in Datastore and Native mode can be used in europe-west2 (London), europe-west3 (Frankfurt) and europe-west6 (Zürich) but also in multi-region mode (europe-west1 (Belgium), europe-west4 (Eemshaven)) which is more expensive.
Therfore there are now intersections you can use to use Google Cloud Functions and Google Firebase Firestore in the same region. If you want to have free egress costs can use europe-west2.
If you don't use it in the same region you can still use it together but there are trans-region egress costs you have to keep in mind. As the warning states you also have to keep latency in mind. The latency issue depends on the needs of your application but is not advised.