Even with using the proxy recommended, firebase kicked out the requests because they weren't to a secured server.

After a lot of headaches, I simply added an (s) to the "http" and viola!  It worked.

Please share your latest build so that I can see the requests.

I updated the script in post #2 so as not to produce entities.
Now it should work correctly.

It works "better" but not in WebGL online.

I made some tests with your last script, I can get/switch the elevations :

- in the editor (in play mode)
- with a windows build (exe)
- with a webGL build offline (in the web browser, not uploaded)

But the terrain remains flat in webGL build once uploaded on our server, while this is what we are looking for...

Any idea ?

Hello.

This is a CORS problem.
https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS
I redirected requests for ArcGIS elevation through a proxy, and now this should work correctly.
A modified script is attached.

Doesn't change anything, I checked the script in my project it is the modified one... Terrain still flat...

Email sent!

This is a CORS problem.
https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS
I redirected requests for ArcGIS elevation through a proxy, and now this should work correctly.
A modified script is attached.

P.S. To reduce response time, it is best to have proxy scripts on your server.
If you want, you can request proxy scripts by email (support@infinity-code.com).