The big green button in the second row of the menu, shown in the image below. It does not appear as a Firefox addon, and Whatsapp itself doesn't seem to have any option regarding this.
Screenshot - Firefox (Click to enlarge image)
And how can I prevent stuff like this getting added to Firefox in the future? This seems to have the same potential for abuse as the desktop browsers' 'toolbars' system, and I wish to preemptively disable such changes to the browser.