Cookies are data that websites store in the browser. This can include anything, including login information. Removing the login means you will have to sign in each time.

FredMcD, thanks for you reply. I'm actually looking for a less general and more technically detailed information on what firefox considers "active logins". I just found hints this option would refer to the content of the temporary credentials cache used for http authentication. I find this the most convincing answer. But others say "active logins" refers to session cookies. Or a mix of both? Or maybe even something completely else? I just don't find reliable technical documentation about this.

I eventually found the documentation I was looking for:

Active Logins: [...] logged in to a website that uses HTTP authentication [...]

• So, clearing the "active login" history will log you out of web sites that use one of the HTTP authentication methods for user login. "Active logins" basically refers to the browsers temporary credentials cache, which is used in these cases. These methods do not use cookies for authentication.
• Clearing the "cookie" history will log you out of web sites that use a cookie based authentication method for user logins. The "cookie" history refers to permanent as well as session based cookies. Session cookies provide (among other things) methods for handling user logins, that are not based on HTTP authentication.

The 2 methods above help website owners to keep users logged in, after they successfully logged in. Both are unrelated to password managers like the "remember passwords" setting in Firefox or Mozilla Lockwise, which help website users while logging in.