hi EveP, this is a feature that has just been in development. i think it is present in firefox 42 beta at the moment - you could give it a try: https://www.mozilla.org/firefox/channel/#beta

A user posted a solution to call up a keyboard for Windows 8.1 on a Surface Pro, using the Charms bar, but I don't know whether it's helpful on Windows 10: https://support.mozilla.org/questions/1041688#answer-713156

hey jscher2000, you can get the icon to invoke the virtual keyboard manually to show by right-clicking the taskbar in win10...