This is another loose end in Firefox relating to cryptography and device attestation on the Web. The latter actually got fixed (ECDSA key storage), so it's clear there's some appetite for improving the current situation.
Allowing users to use TouchID (or FaceID) as available on any other mainstream browser would enable passwordless technology to spread further and enter the mainstream.
If this remains unavailable, then sites looking to offer this option to their users will have no other choice but to provide an alternative, more complex user experience to compensate. Or simply not offer passwordless or 2FA authentication at all.