On your toolbar area right click an empty space and open Customize. Then drag the bookmark icon up to your toolbar area and drop.

Make sure that you do not run Firefox in Full Screen mode with all toolbars hidden.

• Press F11 to toggle full screen mode (Firefox/File > Full Screen)

Thanks for the replies. Firefox is not running in full screen mode. When I drag the bookmarks toolbar from the customize menu, it will only stay displayed if dragged onto the same row as the address bar. When I drag it to create a third row below the address bar, the toolbar continues to hide itself.

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

Press F10 or tap the Alt key to bring up the "Menu Bar" temporarily if the Menu Bar is hidden.

After enabling the bookmarks toolbar under the view menu (F10), the toolbar will still disappear after creating a new tab or opening a new bookmark.

Oh, wow. I just found the issue: the Hide BookmarksBar add on.

What a strange Add-on, why would someone make an Add-on on that is built in to Firefox already. I must be missing something.