What about F-Droid?
Passkeys support is now planned for Firefox 122.
https://connect.mozilla.org/t5/ideas/support-webauthn-passkeys/idc-p/45437/highlight/true#M26276
This version was supposed to bring WebAuthn passkeys support as this source mentioned: https://connect.mozilla.org/t5/ideas/support-webauthn-passkeys/idc-p/39321/highlight/true#M22887
Sadly this is not part of the release notes. I’m disappointed but I hope this will be implemented soon in Firefox.
I saw the term “threadiverse” being used to group all Reddit alternatives interoperable with the Fediverse.