Hi, it's possible the button got removed from the bar and can be added back from the Customize panel. This article will get you to that feature: Customize Firefox controls, buttons and toolbars. Can you find it there and drag it back to the desired spot?

• if missing items are in the Customize palette then drag them back from the Customize window to the toolbar
• if you do not see an item on a toolbar and in the Customize palette then click the Restore Defaults button in the Customize palette window to restore the default toolbar set.

not sure how it disappeared, but finding new functions is always nice! Thank you!