So disable DoH in Firefox, or change the provider? If you know enough to choose a DoH provider, the browser defaults are not very relevant, presumably.
That's great for the handful of people that read Hacker News and understand DNS, it's a trash solution for the rest of the world. Defaults matter because they are rarely changed.
Right now the defaults are unencrypted DNS handled by ISPs which in the US have explicit legal permission to sell your data.