Every user is free to run their own resolver or use any of their ISPs or third parties, which is pretty close to the definition of something decentralized.
Except, as others have pointed out, there are documented cases of ISPs hijacking DNS traffic, even for people who have configured their client to use resolvers other than their ISP, which is possible because of DNS's lack of authentication or encryption.
Besides, I don't see how adding an option for DoH to Firefox is centralizing anything, you're free to set the DoH URL to whatever you like, and you're free to run your own DoH resolver, just like you're free to run your own vanilla DNS resolver.
