It sounds like you’ll have to set your Pihole as the DNS server in Firefox’s settings, and then maybe from there it’ll work itself out? Or maybe the Pihole documentation will be updated in the next few days with some instructions on enabling this. I’m unsure myself to be honest.
I thought if you enable DAITA, you can either connect to a DAITA-Enabled server, or if you don’t, it’ll automatically multihop to a DAITA server. For speed I think you’d want to go straight for the DAITA but I’m fairly certain you can do both if multihopping is ideal.