That extension still works for me (Firefox 40.0.3/Windows 7).

Does your toolbar have an "overflow" button? It looks like a heavy >> and drops down a list of buttons that do not fit on the bar.

If not, could you check whether the button is available in Customize mode on the menu, as an available button, or on a different bar? This article describes how to start up Customize: Customize Firefox controls, buttons and toolbars.

The Customize pane also has a "Restore Defaults" button at the bottom which resets the toolbar button selection to what appears at first installation. If you then Disable and re-Enable the Pin-it extension, I would expect the button to appear.

If that doesn't work either, could there be a conflict with the Amazon extension? You could try disabling that one to see whether it makes any difference.

Thanks for your help. I tried all your suggestions but still couldn't get the Pin-It button to show up anywhere, Finally installed the Share Button for Pinterest from the Mozilla list of available add-ons. Haven't tried it yet but if it doesn't work, maybe I'm not meant to Pin. Thanks again.