On Mac closing Firefox can be more complicated because you can close all open windows, but keep Firefox running with the menu bar visible or minimized in the Doc.

Make sure to use "Quit Firefox" with all windows still open, especially if you want to use "Restore Previous Session". Otherwise check "History -> Restore Previous Session" and "History -> Recently Closed Tabs/Windows".

• https://support.mozilla.org/en-US/kb/restore-previous-session

When I close Firefox I go to the top bar where it shows the program and click "Quit Firefox" much as I do with other programs. If you look at the attached settings screen shot you can see I have the box next to Clear History when Firefox closes, checked but it does NOT clear history so I've been doing it manually.