well mkenduck, its because some providers want to or need to give you new cookies for reasons

Detail like websites remembering you and log you in automatically is stored in a cookie.

You can use these steps to make a website recognize and remember you.

• create a cookie allow exception with the proper protocol (https:// or http://) to make a website remember you

You can check that you aren't clearing important cookies.

• using "Delete cookies and site data when Firefox is closed" keeps cookies with an allow exception
• using "Clear history when Firefox closes" to clear cookies clears all cookies including cookies with an allow exception you may want to keep
• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exceptions for passwords and other website specific data

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: "Manage Exceptions/Permissions"