You can only have one session of firefox open at a time and that session includes all the tabs and windows you had open. When you restored, it should have opened all of them back up for you. Is that what you are referring to?

Maybe he was using Tab Groups?

Check out this article to see how to access Tab groups and maybe you'll find the other session:
https://support.mozilla.org/en-US/kb/what-are-tab-groups

Always make sure to use Firefox > Exit Firefox to close all open windows at once.
Otherwise only the last closed window will be restored.
In that case see if that window is still under "History > Recently Closed Windows"

This is what I did. I had a session open with several tabs, minimized it, and opened Firefox again from my desktop (as an additional session).

cor-el.... I have both sessions back now. I did go to "History>Recently Closed Windows>Restore All Windows", and the missing session came back. Thank you very much for your help!

You're welcome