It sometimes ask about it during view/edit passwords.

If you set the Primary Password then Firefox will ask for this password. *https://support.mozilla.org/en-US/kb/use-primary-password-protect-stored-logins Using the PP is more secure since this password does encrypt the key stored in key4.db, while using Windows to authenticate is merely cosmetic because having access to logins.json and key4.db is sufficient to access the logins. Using Sync can also be a reason. *https://support.mozilla.org/en-US/kb/using-primary-password-sync