I don’t use iCloud Keychain but I noticed this in the release notes for Firefox 122:
Firefox now supports creating and using passkeys stored in the iCloud Keychain on macOS.
Less browser lock-in makes it easier to choose iCloud Keychain instead of paying for a password manager.
Firefox, like Safari, has a built-in translation feature, it got better in Firefox 122.
It does. It also, of course, increases your risk of being locked in to iCloud Keychain. 
If it’s anything like the Chrome functionality it should also allow third-party password managers relying on the native autofill functionality to benefit. That’s my hope, anyway. FWIW I just got done breaking out of iCloud Keychain when moving to Strongbox, a laborious and manual process because passkey export is simply not supported, but I now have all my passkeys under my control, and Safari, Chrome and (soon, hopefully) Firefox will use the stored passkeys.