Since last update my screenshot does not work!!..
The same here. With the first installation of Firefox Quantum it was easy as 1-2-3.
Now: “Keine gespeicherten Bildschirmfotos. Los, erstellen Sie welche.” But how a m i supposed to do this?
You can click the 3 dots in the URL bar to see an option to take a screenshot, or right click on a page and choose Take a Screenshot. Sorry for the confusion.