Screenshot in Unity/Ubuntu within VirtualBox

I'm trying to take a screenshot of Firefox running on Unity/Ubuntu 16.04 inside VirtualBox 5.1.2. Pressing Prt Scrn pops open a little app called **Screenshot** but the screenshot shown is only ever the desktop, with no application windows visible. My host operating system is Windows 10 Pro. Does anyone know how to resolve this and get screenshot functionality working inside VirtualBox?