The issue isn't whether you can choose your resolver for Firefox, it's that it balkanizes the namespace resolution mechanism.
Sure, Firefox is using CDN resolver #1, "optimized for the browser experience", while Spotify uses the CDN resolver #2, "optimized for music discovery".
The namespace will balkanize, and with that the control moves to the owners of the resolvers. That would be a natural evolution of the infrastructure purely due to literal "network effects".
If data can be gleaned from current DNS requests, what data can be gleaned from a browser sending metadata? Who controls those DoH servers?
At least the current DNS namespace, nominally, is devolved, particularly with the explosion of TLDs. That has other disadvantages, but there are advantages too.
NextDNS is great. I've been using it for the last few months to access Handshake sites[1] and there have been no issues, and it's important that there are more resolvers than just Cloudflare and Google on the market.
[1] You need to enable it in your NextDNS settings.
And I think from the context of the parent, you can choose your CDN('s resolver) -- my version of Firefox (77 on macOS) has NextDNS among the default DoH providers.