Check that the Bookmarks Toolbar is visible and that the "Bookmarks Toolbar items" is still placed on the Bookmarks Toolbar.
If the menu bar is hidden then press F10 or hold down the Alt key, that should make the "Menu Bar" appear.

Make sure that toolbars like the "Navigation Toolbar" and the "Bookmarks Toolbar" are visible: "View > Toolbars"

• Open the Customize window via "View > Toolbars > Customize" or via "Firefox > Options > Toolbar Layout" (Linux, Windows)
• Check that the "Bookmarks Toolbar items" is on the Bookmarks Toolbar
• If the "Bookmarks Toolbar items" is not on the Bookmarks Toolbar then drag it back from the toolbar palette in the customize window to the Bookmarks Toolbar
• If you do not see the "Bookmarks Toolbar items" or other items that are missing in the toolbar palette or on a toolbar then click the "Restore Default Set" button

• http://kb.mozillazine.org/Toolbar_customization
• https://support.mozilla.com/kb/Back+and+forward+or+other+toolbar+items+are+missing

"If the "Bookmarks Toolbar items" is not on the Bookmarks Toolbar then drag it back from the toolbar palette in the customize window to the Bookmarks Toolbar"

WOW!!! That did the trick! I would have NEVER thought of doing that (because I had no idea "Bookmarks Toolbar items" even existed)!

Thank you!!!

You're welcome