System DoH is certainly the proper way of handling this so I'll just share my use cases that don't fit it.
Business VPN hijacks DNS configuration and now I'm leaking all requests to my company's (or a 3rd-party). I trust them with my business request but unfortunately we all do personal stuff everywhere and having a specific browser with my specific DoH configuration would avoid that.
Many devices, many physical networks, lots of unpredictability with different configs. Having a specific browser I know is using DoH for sure would give me more peace of mind that I'm not inadvertently leaking DNS.