I don't see how they could enable it by default unless they query both local and DoH at the same time .. or perhaps a hybrid model where they only using DoH by default on certain TLD's and / or domains.

Corporate users would be utterly broken unless their IT staff are managing the proxy settings in all their installed browsers, which is often not the case. This would lead to Security and IT staff blocking CF DNS until it was fixed.

I believe this is a good thing to watch for, but I can't imagine Mozilla not having thought this through.