Correct - that would be visible via ClientHello. But Firefox also enabled ECH (when DoH is enabled) a while back - https://support.mozilla.org/en-US/kb/faq-encrypted-client-he... .

also this: https://support.mozilla.org/en-US/kb/firefox-dns-over-https#...

DoH in Firefox provides you the control to choose when to enable or disable and which DNS provider to choose, while Android does not provide any such choice or even make it known to the user when DoH is used or not. In addition, Firefox only partners with DNS providers that have legally-binding agreements for strongest privacy guarantees - see https://wiki.mozilla.org/Security/doh-resolver-policy .

Yes, this. AND while Firefox is providing you the control to choose when to enable or disable DoH, you don't get that control at OS-level, or even the visibility of what the OS is choosing on your behalf for each such query.