Try using the Customize feature to drag it back to the desired location.

(Although you can right-click > Move to Toolbar, you will have to use Customize anyway because probably you don't want the double-button at the far right end of the bar.)

This article should lead you to it: Customize Firefox controls, buttons and toolbars.

Does that work?

Actually, if you meant "Bookmarks Toolbar Items", you'll also need to make sure the Bookmarks Toolbar (empty though it may be) is displayed. If you don't see the thin/empty bar below the main navigation toolbar, you can display it using either:

• right-click the "+" button on the tab bar
• Show/Hide Toolbars