If you close all Firefox windows bar one, close all tabs except an empty one, then quit the last Firefox window via the menu, and then start Firefox in Safe Mode, does this still occur? https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

The only notices I've gotten regarding add-on updates have come from the Extensions themselves, not Firefox. Could you take a screenshot showing the full browser if this still occurs in Safe Mode?

You can check for problems with preferences.

Delete possible user.js and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

There is software like Advanced SystemCare with its Surfing Protection feature that can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.