When you select Keep Me Signed In, the site usually sets a persistent cookie. Persistent means it does not expire when you shut down Firefox but instead is set to expire at some future date.

Cookies can be cleared in a wide variety of ways. For example, you could set cookies to expire when you shut down Firefox, overriding the site's attempt to make it persistent. Or you might use Firefox's feature to clear private data at shutdown. Or an add-on might delete cookies. Or external privacy/utility software might do it.

Let's start with Firefox's privacy settings. This article describes the settings that give you the broadest compatibility: Websites say cookies are blocked - Unblock them. But that might not be your preference.

How would you like to manage cookies?

A detail like a website remembering you (log you in automatically) is stored in a cookie.

You can use these steps to make a website remember you:

• Create a cookie 'allow' exception to keep such cookies, especially in case of secure websites and when cookies expire when you close Firefox.
• Tools > Options > Privacy > "Use custom settings for history" > Cookies: Exceptions

In case you use "Clear history when Firefox closes" or otherwise clear history.

• do not clear the Cookies
• do not clear the Site Preferences

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/remove-recent-browsing-search-and-download-history

Clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, passwords, and other website specific data.

Clearing cookies will remove all specified (selected) cookies including cookies with an allow exception that you would like to keep.

You can remove the cookies of a website to make the website forget about you.