Do you mean you have a drop-down list from a toolbar button on the location/address bar or do you have the bookmarks sidebar (Ctrl+B) ?

Make sure toolbars like the "Bookmarks Toolbar" are visible. Open the Customize window and set what toolbars (Show/Hide Toolbars) and toolbar items to display.

• Right-click empty toolbar area -> Customize
• "3-bar" menu button -> Customize
• View -> Toolbars
  *you can tap the Alt key or press the F10 key to show the hidden Menu Bar temporarily

• check that "Bookmarks Toolbar Items" is on the Bookmarks Toolbar
• if "Bookmarks Toolbar Items" is not on the Bookmarks Toolbar then drag it back from the Customize palette to the Customize window to the Bookmarks Toolbar

• if missing items are in the Customize palette then drag them back from the Customize window to the toolbar
• if you do not see an item on a toolbar and in the Customize palette then click the Restore Defaults button in the Customize palette window to restore the default toolbar set.

See also:

• https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars

The ONLY thing on my bookmarks bar, having performed the instruction within 'Customize', is a folder icon plus the words "Bookmarks Menu".

By left clicking on that item, all my bookmarks appear in a dropdown menu. ********************************* I CAN slide the bookmarks out from the dropdown list INTO the horizontal area of the bookmark bar where they belong. Since I have over a hundred bookmarks, moving each from the dropdown list to the horizontal list becomes a horrendous chore, best left for a computer to accomplish.