Your ‘logged-in’ status is stored in special cookies. If the cookie is removed, you are logged out.

Type about:preferences#privacy<enter> in the address bar. The button next to History, select Use Custom Settings.

At the bottom of the page, turn on Clear History When Firefox Closes. At the far right, press the Settings button. Turn on ONLY Cache and Form And Search History leaving the others off.

This has been happening to me for several months now. No matter what settings I use, nothing keeps me logged in to all my sites, like it USED to do! I have checked, re-checked and re-checked (again) ALL the settings telling Firefox NOT to log me out of sites, all to no avail. This happened for several weeks in November 2019 and then, without any changes, it quit happening! I stayed logged in to all the sites on my PC laptop and life was good. Until 2 or 3 weeks ago (in January 2020), when it began logging me out of every site! In fact, now I can log into Yahoo or Facebook, go to check the new at CNN and when I come back to Facebook, in the SAME session, I have been logged out! Why????? Help!!

If you use external cleanup software (CCleaner) then check the settings for Firefox.

What are the current settings in Firefox for keeping cookies and clearing personal data like cookies?

Make sure you do not use Private Browsing mode.

• https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history
• https://support.mozilla.org/en-US/kb/common-myths-about-private-browsing

You can let the cookies expire when Firefox is closed to make them session cookies instead of using "Clear history when Firefox closes" to clear the cookies.

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: [ ] "Delete cookies and site data when Firefox is closed"

You can create an allow exception to keep specific 'remember me' cookies to log you in automatically.

In case you use "Clear history when Firefox closes" or otherwise clear history.

• do not clear the Cookies
• do not clear the Site Preferences

• Options/Preferences -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exception for password and other website specific data
• clearing "Cookies" will remove all selected cookies including cookies with an allow exception you may want to keep

It is possible that the cookies.sqlite file in the Firefox profile folder that stores the cookies got corrupted.

• rename/remove cookies.sqlite (cookies.sqlite.old) and when present delete cookies.sqlite-shm and cookies.sqlite-wal in the Firefox profile folder with Firefox closed in case cookies.sqlite got corrupted.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/Profiles

I will try all this again. Thanks for responding quickly! Something I wasn't expecting! I will report back tomorrow on what happened.

OK. I think it is fixed, for now. I had to work with the "cookies.sqlite" files.

I did this: rename/remove cookies.sqlite (cookies.sqlite.old) and when present delete cookies.sqlite-shm and cookies.sqlite-wal in the Firefox profile folder with Firefox closed in case cookies.sqlite got corrupted.

So, maybe those files were corrupted.

Again, thanks for this answer as it has seemed to clear up my issues!!