From MozillaZine Knowledge Base
key3.db contains a key used to encrypt and decrypt saved passwords.
Manually editing key3.db is not possible.
This file can be moved to a different profile, but there is no point in doing so unless signons.txt (Firefox, Sunbird) or (number).s (Mozilla Suite, Thunderbird) is also moved.
Deleting key3.db will render your saved user names and passwords unretrievable.