Maybe, but no one is forced to use their ISPs DNS either. Giving user's data to one snoop to protect them from another doesn't sound like a win to me either way.
The nice thing about Firefox is that you can configure it to use any DNS resolver you please!
It may perhaps be worth considering privacy-enhancing settings in the context of a mass-market product, where the vast majority of users do not know what DNS is and do not run their own resolvers. In such a context, do you think it might be possible that most users might be better off with privacy-first defaults? With, of course, all the appropriate configuration options for more sophisticated users.
Any such feature, of course, must be something configurable such that any user wishing to use their own DNS provider can do so.