What's your computer system and Firefox version?

In the event that the passwords information is lost, you should record all information in a separate text file somewhere else on your hard drive, or written down. You can easily copy and paste if you need to do so. If you are concerned about someone else looking at that file, you can compress it using a password.

What about the rest of your data/settings? Any problem with them?

Look on your desktop. Do you see a folder called; Old Firefox?

Type about:profiles<enter> in the address bar. How many profiles are listed? How many should be there?

I think we need to distinguish between two situations:

(1) Sites where you log in and check a box to stay logged in, and allow Firefox to save your login; and (2) Sites where you log in and check a box to stay logged in, but you do not save your login in Firefox's password manager.

In both scenarios, the site sets a persistent cookie indicating that your browser is a known user. Firefox retains that cookie and sends it back to the site with every request. Such cookies usually have an expiration date, possibly 6 months from login for sites that do not handle sensitive data, it varies. If that cookie is cleared and Firefox doesn't send it back to the site, the site will force you to log in again before accessing protected pages.

After the cookie is cleared, in Scenario #1, Firefox can fill the login form and/or automatically log in for you. In Scenario #2, you need to enter the login yourself.

There are many possible reasons that cookies might get cleared in Firefox including privacy settings, using the Clear Recent History feature, add-ons that touch cookies, and external programs (e.g., security/privacy software, CCleaner) that clean up browser data from time to time. And sometimes Firefox's cookie database may become corrupted.