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

Thanks. I think I tripped over part of the answer. So why was this feature removed? It's disconcerting when the new download invalidates features of the old!

It's a change I disagree with, but the Firefox user interface team decided to remove it citing parity with chrome and IE. There have been requests to add the drop-marker back but these have been declined.