How do I remove the 'Whatsapp' button in Firefox's menu? 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.
Whatsapp is the last app you used to share a link. 
To change it to something else, you need to share a link with another app, or if you want to remove any app icon from the menu, just uninstall and install the app again.