That's up to Passkeys to support the Browser not the other way around. Did you contact Passkey support about Browser support?

There is a bug report about supporting passkeys in Firefox. *[https://bugzilla.mozilla.org/show_bug.cgi?id=1765549 1765549] - (webauthn-lvl2) [Meta] WebAuthn Level 2 changes and features *[https://bugzilla.mozilla.org/show_bug.cgi?id=passkeys passkeys] - (passkeys) Implement support for synced application credentials (passkeys) (<i>please do not comment in bug reports<br>https://bugzilla.mozilla.org/page.cgi?id=etiquette.html</i>)