Is there an "overflow" icon on the bar where it might now be shown? The button looks something like >>. To move it back into view you can use the Customize feature.

The Customize feature is also where you would check for toolbar buttons and controls that have been removed from all bars. They should be available to drag back to the bar.

This article should get you started: Customize Firefox controls, buttons and toolbars.

Does that work?

You may have accidentally removed button(s) from the Navigation Toolbar.

If you right-click a toolbar item in normal (non-customize) mode then you can remove this item from the toolbar.

• "Remove from Toolbar" will remove this item from the Navigation Toolbar and moves it to the Customize palette ("3-bar" Firefox menu button > Customize)
• "Move to Menu" will move this item to the "3-bar" Firefox menu button drop-down list
• "Move to Toolbar" in the "3-bar" Firefox menu button drop-down list will move this item to the toolbar