If it was guaranteed that say FIDO would stop sites demanding phone numbers to do snake oil "2FA", then I'd be fully supportive. But from what I've witnessed surveillace companies won't take steps that don't benefit them - why would they ever stop demanding phone numbers now that they can? And so whatever it turns out being, I can only imagine that "passwordless" will be similarly abused ("never let a good crisis go to waste"). At the very least it will create another configuration speed bump that interrupts workflows based around trying to compartmentalize the surveillance industry (eg I only run web browsers in VMs).
If you really want to drive adoption of FIDO, then instead of pushing hardware tokens (and corresponding heavyweight process), Mozilla should create a software FIDO key within the browser. That is something that I can see evolving features that help users rather than restrict them. It would then be up to an end user whether to use a hardware token in place of that, iff they determine that a given account is that important.