Linux Passkey Manager od Leo S.

Linux Passkey Manager adds native passkey support to Firefox on Linux, where no built-in platform authenticator exists. It works on any website that supports the WebAuthn standard and requires no additional software or hardware.
When a website asks you to create a passkey, the addon generates a P-256 key pair directly in your browser using the Web Crypto API, stores the private key locally in your browser profile, and returns a valid WebAuthn attestation to the website. When you sign in with a passkey, the addon finds the matching credential, signs the server challenge with your stored private key, and returns a cryptographically valid assertion.
If no passkey is stored for a website, the addon stays completely silent and lets the browser handle the request normally. A small confirmation dialog appears only when a passkey action is actually available. It adapts to your system's dark or light mode and also works on websites that show their login in a modal dialog. Through the toolbar popup you can view all saved passkeys, see when they were created and how often they have been used, and delete them at any time.
Backup and sync: Removing the addon also removes its stored passkeys, so you can export them to a backup file and import them again later. If you use several computers, you can sync your passkeys through your own Nextcloud. Each computer merges its passkeys with the synced file, so new passkeys, sign-in counters and deletions show up everywhere. Backup files and the sync file are always encrypted with a password you choose (AES-256-GCM with a PBKDF2 key), so neither the file nor your Nextcloud server can reveal your private keys without that password.
Your private keys are stored in your local browser profile. The addon has no external dependencies, collects no data, requires no account, and contacts no server of its own. The only network connection it ever makes is to your own Nextcloud, and only if you turn on sync.