Do you mean names and passwords in the Password Manager or do you mean that you are no longer logged on to (remembered by) websites after closing and restarting Firefox?

• Websites remembering you and automatically log you in is stored in a cookie.
• Create an allow cookie exception (Tools > Options > Privacy > Cookies: Exceptions) to keep such a cookie, especially for secure websites and if cookies expire when Firefox is closed.
• In Private Browsing - Use Firefox without saving history mode all cookies are session cookies that expire if that session is ended, so websites won't remember you.
• Do not use Delete browsing, search and download history on Firefox to clear the "Cookies" and the "Site Preferences"

Clearing "Site Preferences" clears all cookies, images, pop-up windows, software installation, and password exceptions.

• http://kb.mozillazine.org/Cookies
• http://kb.mozillazine.org/Password_Manager
• http://kb.mozillazine.org/User_name_and_password_not_remembered

"do you mean that you are no longer logged on to (remembered by) websites after closing and restarting Firefox?"

Yes. For instance I have had to log into this site (yet again today) to reply you.