[mstolpm]

You can already "autofill" Firefox (and other) input fields with keychain data in Sonoma. Its a bit cumbersome, but right click into the field, select "autofill" from the context menu, then either "passwords" or "contacts", search/select/confirm the data to be inserted.

[samuria]

There's a bunch more to "autofilling". 1Password has ability to generate new passwords, save new registrations, autofill MFA codes, and even read QR codes on screen to setup MFA. None of this is possible on Firefox with Keychain.