Hi vanvamat, glad to see you on Mozilla Support Forum.

You can fix this problem by following steps:-

• Click on menu icon.
• Click on options.
• In left pane, you will find Privacy & Security menu, click on it.
• In this menu, you will find an option named Cookies and Site data.
• Just uncheck Delete cookies and site data when Firefox is closed.

If it doesn't solved your problem, then feel free to ask in Reply Section. Thanks for raising your question in Mozilla Support Forum.

The 'delete cookies and site databox was not checked

A detail like a website remembering you (log you in automatically) is stored in a cookie, so you need to make sue that you keep these special cookies.

You can create an allow exception to keep cookies for specific websites.

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: "Manage Permissions"

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