Firefox >Tools > Settings > Privacy & Security >Saved Passwords > + [Add password]

[AlexanderNevsky]

Add to stored password records -- and to the [Add password] dialogue -- a field entitled "Notes" in which the user can store important information related to the given URL.

For example, a "Notes" field for a credit-card password could store the credit-card's security code (CVC) and the fee that the bank charges for cash advances.

The capacity of the suggested "Notes" field should be about 500 characters. 

[Jon]

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[Jon]

Potentially similar idea here: Add custom fields in Password Manager