Hi, please see: Restore previous session - Configure when Firefox shows your most recent tabs and windows. This also tells you how to reset your home page.

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Firefox only shows tabs that were closed - which was a lot - but not what was open during previous day session

tabs that were open a week before and never closed - how to find them?

The default session store feature saves one session - that's it. If you want multiple sessions saved you need a Session Manager extension. https://addons.mozilla.org/en-US/firefox/addon/session-manager/

If you happen to install or update an add-on such as Skype that requires a Firefox restart, you need to do the session restore upon restart of Firefox or you'll lose the previous session when you close Firefox.

And when your homepage gets reset (thus losing the Restore Previous Session button at the bottom of the default homepage) you need to restore the previous session via the History menu; and you need to do that before you close Firefox.