@dalias I think that there is some integration with your "OSKeyStore":
Some references to this in automated tests:
https://searchfox.org/mozilla-central/source/toolkit/modules/tests/browser/browser_CreditCard.js
I found someone referencing this in troubleshooting as well: https://www.codejam.info/2022/05/firefox-credit-card-autofill-not-working-on-linux.html