Firefox and my views on the tradeoffs of using DNS over HTTPS
For those who have not heard, Mozilla is (still) planning to have Firefox support and likely default to resolving DNS names through DNS over HTTPS using Cloudflare's DoH server (see eg this news article). The alternate, more scary way of putting this is that Mozilla is planning to send all of your DNS lookups (well, for web browsing) to Cloudflare, instead of your own ISP or your own DNS server. People have mixed feelings about Cloudflare, and beyond that issue and the issue of privacy from Cloudflare itself, there is the fact that Cloudflare is a US company, subject to demands by the US government, and the Cloudflare DoH server you wind up using may not be located in your country and thus not covered by laws and regulations that your ISP's DNS service is possibly subject to (such as Europe's GDPR).
Combining this with that fact that today, your large ISP is one of your threats creates a bunch of unhappy tradeoffs for Mozilla for deploying DNS over HTTPS in Firefox. On the one hand, some or many people are being intruded on today with ISP surveillance and even ISP tampering with DNS results, and these people will have their lives improved by switching to DoH from a trustworthy provider. On the other hand, some people will be exposed to additional risks they did not already have by a switch to DoH with Cloudflare, and even for people who were already being intruded on by their ISP, the risks are different.
Pragmatically, it seems likely that turning on DoH by default in Firefox will improve the situation with DNS snooping for many people. Mozilla has a contract with Cloudflare about DNS privacy, which is more than you have with your ISP (for typical people), and Cloudflare's POPs are widely distributed around the world and so are probably in most people's countries (making them at least partially subject to your laws and regulations). I suspect that Mozilla will be making this argument both internally and externally as the rollout approaches, along with 'you can opt out if you want to'.
However, some number of people are not having their DNS queries snooped today, and even when people are having them intruded on, that intrusion is spread widely across the ISP industry world wide instead of concentrated in one single place (Cloudflare). The currently un-snooped definitely have their situation made worse by having their DNS queries sent to Cloudflare, even if the risk of something bad happening is probably low. As for the distributed definite snooping versus centralized possible snooping argument, I don't have any answer. They're both bad, and we don't and can't know whether or not the latter will happen.
I don't pretend to know what Mozilla should do here. I'm not even sure there is a right answer. None of the choices make me happy, nor does the thought the DoH to Cloudflare by default is probably the pragmatically least generally harmful option, the choice that does the most good for the most people even though it harms some people.
To put it another way, I don't think there's any choice that Mozilla can make here that doesn't harm some people through either action or inaction.
(This sort of elaborates on some tweets of mine.)