Hey there! Clearing the cache deletes cookies and other temporarily saved data which can be used as an authentication without directly storing any password information. You can save password information directly in Firefox, did you do that? Do you still have your passwords? If you just want Firefox to ask you again if you want to save passwords, follow along and go to Settings > Privacy & Security > Logins and Passwords and check the box next to "Ask to save logins and passwords for websites"