Linux people: is there a passkeys implementation that does not use physical hardware tokens (software only), is open source, works with Firefox, and allows credentials to be backed up and copied to other devices by hand, without going through some cloud service? I don't think I'm asking for much, but this is what I consider the minimum for me actually using passkeys. I want to be 100% sure of never losing them because I have multiple backups and can use them on multiple machines.
