This article will guide you to the Passwords page where you can delete the saved logins, and then you can turn off the feature to ask you whether to save them:

Password Manager - Remember, delete and edit logins and passwords in Firefox

Does that work for you?

Sorry, forgot to mention: if you are currently signed in to a website, and your session is being kept alive by a cookie, removing saved logins won't sign you out of that site, you'll need to sign out yourself if you want to close out that session.

If you want to start with fresh password files then you can remove logins.json (encrypted usernames and passwords) and key4.db (encryption key and primary password) in the Firefox profile folder. Note that you need to set a new Primary Password if you remove key4.db.

• https://support.mozilla.org/en-US/kb/use-primary-password-protect-stored-logins

This is different than websites remembering you and log you in automatically as that is stored in session ID cookies, so you need to remove cookies for specific websites to make the website forget about you.