In Google Firebase pricing (https://firebase.google.com/pricing/) table states:
The Spark plan only allows outbound network requests to Google-owned services.
Does it mean that JS function in web browser (client) page can't call Firebase function and retrieve data in FREE TIER?
The pricing tag has a question mark, it says that The Spark plan allows outbound network requests only to Google-owned services. Inbound invocation requests are allowed within the quota. On the Blaze plan, Cloud Functions provides a perpetual free tier. The first 2,000,000 invocations, 400,000 GB-sec, 200,000 CPU-sec, and 5 GB of Internet egress traffic is provided for free each month.
If someone sends requests to your functions, these requests are inbounded requests. And It does have a 5 GB egress qouta per month. However, if you have to use other services, like fetch other url outside google, is prohibited in Free tier.
Unless your client was using some Google service as a proxy, or you change you design for the web server to pull the data from a google hosted site, it will not work from a client on your network.
I have played a bit with their Nest thermostat dev tools, which use firebase. They would be allowed to use the free tier as a backend database, as their dev tools are run in their cloud.
