Users can test their ESNI support online here: https://www.cloudflare.com/ssl/encrypted-sni/
But, I encourage you to try to get it working in _your_ firefox, where that site says you are in fact using ESNI. It is trickier than just enabling a flag.
If you think this means people are using it, sure. Firefox has ~9% of market share, and a very small percent (maybe 0.1%) of those users have correctly configured this, and when those users visit a page hosted by cloudflare they will may be use ESNI. That is in fact more than zero users. It is a very very small number today, but hopefully it will grow in the future.
And while that page you link tests ESNI, I cannot verify that any other domain supports ESNI. I am unaware of any tooling that lets me test that today.
Only if you use their built-in DoH resolver. The public keys for ESNI are distributed by DNS records and as I understand it there's a bunch of work to be done for Firefox to retrieve these records from classic DNS servers. That work has been classified P5 (we won't do it, but might accept patches)[0].