My understanding is that anyone running Firefox already has, unless they went out of their way not to. Is that no longer the case?
"People" means common folk, with their Edge, Chrome, Android Chrome and iOS Safari. There is the web that 'just works' and the web where you need to open up the back of the unit and start swapping wires.
Common folk delegate via their network administrator, often an ISP.
Yes exactly.
If you mean DoH it's still pointing to the DNS administered by ICANN/IANA just a different entry point.