You can get the drop down list by either right-clicking on the back/forward buttons, or holding down the left button until the list appears.

If you want the drop-down arrow you can add it with the Back/forward dropmarker add-on - https://addons.mozilla.org/firefox/addon/backforward-dropmarker

I don't mean a drop down arrow, I need the back and forth arrows so that when I am in a screen and want to go back to the previous screen that is the missing arrow in my toolbar.

In that case this link should help you restore them - https://support.mozilla.com/kb/Back+and+forward+or+other+toolbar+items+are+missing

Right click a blank part of the Tab bar (or the Home button) and go to Customize. In the panel which appears, click the button called "Restore Default Set".

If that doesn't work, click Help | Restart with Add-ons disabled, then checkmark the option called Reset toolbars and Controls. Then click "Make changes and restart".