At home I've got a pihole handling my DNS, including using DoH to Cloudflare.
I assume that this configuration is superior to whatever FF is doing natively, and I should disable FF's DoH support?
yeah there is no reason (I can think of) why you need FF DoH with your setup. In fact if you were to enable DoH in FF it would bypass your pie-hole - so you most certainly want to avoid that.
my setup looks pretty much like yours with some additional /etc/hosts blocking[1] on the client just to avoid the round-trip to the pie-hole. it's also a double insulation (but it's more of a performance reason than bc of paranoia). I found that switching off ipv6 dns resolution in FF (`network.dns.disableIPv6` in about:config) has tremendously sped up my DNS lookups in FF (though I haven't had time to analyze why).
in case you're worried about homograph phishing attacks you could also add a regex to your pie-hole's dnsmasq (not sure what piehole uses but I know it has a fork of dnsmask that supports regex) so that punicode domains (any domains matching "xn--") are sinkholed to 0.0.0.0 as well.