Hi, the button has probably gone back into the customize menu from where you can drag it out to a toolbar in the location of your choice, so please see - Customize Firefox controls, buttons and toolbars. If you can't see it, try clicking on Restore Defaults.

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Found the little sucker in Customize..thanks

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

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