The Floppy symbol does not appear on the upper RIGHT corner, instead an empty invisible icon appears.
Clicking on the invisible icon does nothing.
Disclaimer: I am using Sandboxie 5.20 to run my firefox 57.0.3

Edit: It must be Sandboxie's fault. Outside Sandboxie, the Firefox plugin works normal and even quite good, and yes the Icon appears! On the upper right corner!