It is very easy to do it.
Just write in the URL bar of your browser: “about:home“.
That is the actual URL for the default Firefox start page.
Once there, check the bottom part of the page where is saying “Restore Previous Session”.
Click that button and the tabs from the previous session will show up.
Be sure not to close your browser and open it back as you will be able to restore only the last session, not more than the last one.