In Firefox's address bar (location bar), enter about:sessionrestore

If there is a session that it's able to restore, the tabs will be shown on this page.

If you use Delete browsing, search and download history on Firefox in Firefox 3.5/6 or Clear Private Data in Firefox 3 to clear the 'Browsing History' when Firefox is closed then restoring tabs from the last session (e.g. "Save & Quit" or "Show my windows and tabs from last time") doesn't work.

If you still don't want browsing history kept beyond the last session, untick "Remember my browsing history for ... days" instead of ticking "Clear history when Firefox closes"