You may have corrupt sessionstore file(s). Delete all sessionstore* files and the sessionstore-backups folder.

Type about:support<enter> in the address bar.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the above file. Then rename or delete it. Restart Firefox.

Note: It would be better to set up an Administrator computer profile for yourself, and a separate non-administrator for others to use.

FredMcD, thank you very much. It worked beautifully. However, only until next start of the laptop. The problem once again is there.

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

Hi Dara_Koper, do you use CCleaner? If so:

Can you go into its settings and deselect the option to clear your Firefox session data? Firefox 56 changed the format and name of the session history files, and this seems to have created a temporary compatibility issue with the cleaning feature that somehow is causing this problem.

If you do not want Firefox to store session history -- it's primarily used for crash recovery, but also allows you to use History > Restore Previous Session -- you can set Firefox to flush all history when you end your Firefox session. Be careful not to clear "Site Preferences" (this includes permissions you grant) or other categories that you want to retain. See: Delete browsing, search and download history on Firefox - the "How do I make Firefox clear my history automatically?" section.

Thank you, both. Solved

Glad to help. Safe Surfing.