Ugh, was wondering why Firefox kept showing intrusive af ads, even though it should be going through the local pi-hole. Turns out, an update made Firefox default to using DoH against Cloudflare's public servers.
First, screw Cloudflare, and screw Firefox for defaulting to using them.
Second, that's just a bad thing to silently turn on — I have my DNS set up the way I like it for a reason, and am not in love with Firefox just deciding it knows better.